A Wearable Fiber-Optoacoustic Interface for Continuous Deep-Tissue Hemodynamics in MRI Environments

A Wearable Fiber-Optoacoustic Interface for Continuous Deep-Tissue Hemodynamics in MRI Environments | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article A Wearable Fiber-Optoacoustic Interface for Continuous Deep-Tissue Hemodynamics in MRI Environments Zhixuan Hu, Minghao Ma, Chaoneng Wu, Xue Bai, Wei Li, Zhongyuan Cheng, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8635823/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Continuous monitoring of arterial waveforms is critical for assessing cardiovascular status in intensive care and intraoperative settings. However, conventional modalities relying on electronics or metallic transducers are strictly incompatible with the strong electromagnetic perturbations in clinical settings, especially magnetic resonance imaging (MRI) suites, creating a blind spot in patient monitoring. Here, we present a fully metal-free, all-fiber optoacoustic system (FOAS) that bridges this gap by integrating focused optical ultrasound generation with ultrasensitive fiber-laser detection in a compact wearable platform. This architecture enables beat-to-beat reconstruction of blood pressure waveforms with high fidelity, preserving morphological features essential for vascular compliance analysis. In-vivo validations, including physiological perturbations (exercise, caffeine) and measurements across 10 healthy volunteers, demonstrated the system's robustness in tracking hemodynamic dynamics and resolving inter-subject waveform variability (e.g., systolic-to-dicrotic notch interval). Crucially, the artifact-free operation was demonstrated inside an active 3T MRI scanner, confirming superior electromagnetic immunity. This work establishes fiber optoacoustic as a transformative platform for ambulatory hemodynamic monitoring, extending precise cardiovascular profiling into electromagnetically constrained clinical environments. Optics/Lasers Wearable devices Optoacoustics Fiber-optic sensors MRI compatibility Hemodynamic monitoring Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Introduction Continuous hemodynamic monitoring--encompassing pulse waveform morphology, beat-to-beat blood pressure (BP) dynamics, and heart rate variability--is a cornerstone of intraoperative management and critical care [1, 2]. Currently, the clinical "gold standard" relies on invasive arterial catheterization, which couples intravascular pressure directly to an external transducer via a fluid-filled column. While offering precision measurement, this invasive approach carries inherent risks of infection, thrombosis, and nerve injury, strictly limiting its use to high-risk surgical patients. For the broader population, non-invasive alternatives are employed but face significant limitations. Oscillometric cuffs, though ubiquitous, provide only intermittent snapshots of cardiovascular status, missing rapid hemodynamic fluctuations [3-5]. Emerging wearable ultrasound patches have recently achieved deep-tissue imaging capabilities by utilizing piezoelectric arrays to transmit and receive acoustic waves [6-9]; however, their fundamental reliance on metallic interconnects and conductive circuits renders them strictly incompatible with Magnetic Resonance Imaging (MRI). Standard ultrasound probes rely on piezoelectric arrays and conductive interconnects, presenting significant hazards in MRI suites due to magnetic attraction forces and radiofrequency (RF) induced heating. Furthermore, these devices introduce susceptibility artifacts that degrade MRI image quality. To address this challenge, optical sensors have emerged as a promising alternative by leveraging the intrinsic electromagnetic interference (EMI) immunity of photons. Unlike electrons, photons are unaffected by magnetic fields, and optical fibers offer the unique capability of low-loss remote signal transmission over at least tens of meters. This allows for a paradigm where a completely passive, all-optical probe operates within the MRI isocenter, while all sensitive electronic interrogation units are safely housed outside the shielded room. Conventional optical methods, such as Fiber Bragg Gratings (FBG) [10-13] or Photoplethysmography (PPG) [14-17], have attempted to exploit this advantage. However, these modalities predominantly operate as contact-based strain gauges or volumetric sensors, inferring vascular dynamics from indirect skin surface deformations or capillary blood volume changes [18]. From a biophysical perspective, the skin and subcutaneous tissues act as a viscoelastic low-pass filter, effectively damping the high-frequency mechanical transients generated by the arterial pulse. Consequently, while these surrogates can track heart rate, they often fail to capture the fine morphological features of deep clinical reference arteries that are essential for precise vascular compliance analysis. To bridge this fundamental gap, we introduce 'Ultrasound on Fiber,' a concept that synergizes the deep-tissue resolving capability of ultrasound with the electromagnetic transparency of fiber optics. Although pioneering studies have demonstrated the feasibility of encoding ultrasonic signals onto optical carriers—exemplified by applications such as ultrasound structural imaging and M-mode interventional intracardiac imaging—translating this technology into robust clinical wearables for blood pressure (BP) waveform dynamics remains challenging. Current approaches are constrained by the sensitivity required for single-vessel deformation detection and the necessity for a metal-free design; consequently, a device capable of continuous BP waveform monitoring in strong magnetic fields, such as MRI, has yet to be demonstrated [19-21]. While high-Q optical resonators offer superior sensitivity, they suffer from acute environmental instability. Conversely, robust low-Q alternatives require either prohibitive interaction lengths or multiple ultrasonic emissions to achieve a viable signal-to-noise ratio for single-vessel surface displacement. To overcome that challenge, we present a fully metal-free Fiber-Optoacoustic System (FOAS) that introduces an active detection architecture. By leveraging focused photoacoustic generation with a fiber-laser sensor, our system achieves outstanding acoustic sensitivity and stability enabled by self-heterodyne detection within a compact footprint. By directly interrogating arterial wall dynamics, the FOAS reconstructs high-fidelity BP waveforms. This capability is validated through multi-subject trials (n=10) and, crucially, safe and artifact-free operation within a 3T MRI scanner, establishing the FOAS as a transformative platform for seamless cardiovascular profiling in electromagnetically constrained environments. System Concept & Characterization 1.1 Principle of Optoacoustic Arterial Diameter Tracking Cardiac ejection generates pulsatile pressure waves that drive cyclic arterial diameter variations, thereby encoding hemodynamic data such as BP dynamics, vascular compliance, and wave reflection [22-25] . To quantify this, our fiber-optoacoustic system (FOAS) utilizes ultrasound M-mode [26, 27] to capture wall echoes via a fiber laser sensor (Fig. 1a). Upon transmission, the focused ultrasonic waves interact with the vessel's upper and lower walls, where acoustic impedance mismatches with the surrounding tissue cause partial signal reflection. By analyzing the time-of-flight of these echoes relative to the speed of sound in tissue, we reconstruct diameter waveforms at a rate of 500 Hz with micrometer precision. Significantly, these diameter waveforms encapsulate rich physiological data, including heart rate, blood pressure, and associated hemodynamic metrics. To achieve MRI compatibility via electromagnetic immunity, we developed a compact (20 × 18 × 10 mm³), metal-free all-optical probe for wearable monitoring (Fig. 1) that seamlessly integrates optoacoustic excitation with fiber laser sensing. The system utilizes a spin-coated functional layer to convert 532 nm laser pulses, delivered via multimode fiber, into ultrasound through the photoacoustic effect [28, 29]. This concave source generates a stable 20 MHz bandwidth signal with a threefold pressure increase compared to planar counterparts, creating a 1.4 mm × 0.5 mm focal zone at a depth of 7 mm to target the radial artery (see Methods). Ultrasound echoes are detected at the skin surface using a 125 μm cylindrical fiber laser sensor with a self-delayed interferometric readout. Unlike conventional dual-frequency schemes, this configuration eliminates the need for dual-frequency polarized laser outputs and polarization-maintaining fibers, thereby simplifying the system architecture and enhancing stability. Furthermore, this approach recovers low- and high-frequency components typically suppressed in dual-frequency setups, effectively extending the bandwidth and improving both resolution and signal-to-noise ratio (SNR). Featuring a broad detection bandwidth of 20 MHz and a noise-equivalent pressure of 0.5 mPa/√Hz, the sensor ensures high-SNR ultrasonic detection (see Supplementary Materials). Currently, the overall system bandwidth is primarily limited by the thickness of the photoacoustic generation coating. 1.2 Signal Processing and Data Acquisition The fiber laser ultrasound sensor, whose output is photodetected and digitized by a field-programmable gate array (FPGA)-based acquisition system (sampling rate 500 MHz, 14-bit resolution). Raw ultrasound signals undergo three processing stages: (1) The anterior and posterior wall signals are enhanced using a deconvolution algorithm. (2) The positions of the anterior and posterior walls are extracted via a peak-finding algorithm. (3) Diameter calculation from the time-of-flight difference Δt between wall echoes: d(t) = c· Δt / 2, where c = 1540 m/s is the assumed sound speed in soft tissue. Real-time display of diameter waveforms (M-mode) is achieved at 500 Hz frame rate, with beat-to-beat BP estimation performed via a compliance-based model (see Methods). 1.3 Wearable Integration For wearable deployment, the all-fiber probe is integrated into a 3D-printed housing designed for radial artery monitoring (Fig. 1a, right inset). The housing features an anatomically contoured base that conforms to the wrist curvature, with an adjustable strap enabling watch-like fixation to maintain stable probe-to-artery alignment during physiological motion. The focal length of the optoacoustic source is 7 mm, optimized for radial artery depth in adult subjects (3–5 mm beneath the skin surface). PVC Wrap at the skin interface distributes contact pressure uniformly while providing acoustic coupling via ultrasound gel. This compact, lightweight form factor (total mass <15 g) allows continuous monitoring across diverse scenarios without restricting limb movement or causing discomfort during extended wear. We have opted to employ the radial artery for dynamic hemodynamic measurements. Its superficial location and clear blood flow signals make it an ideal candidate for such measurements. Moreover, the radial artery exhibits a relatively stable anatomical structure, with minimal variations in its course and position across different individuals. This not only simplifies the measurement process but also enhances the standardization of the procedure, thereby ensuring the accuracy and reproducibility of the measurement outcomes. 1.4 System Integration and MRI Compatibility All optical and electronic components containing ferromagnetic materials are located outside the MRI bore, with only the wearable probe and optical fiber cables entering the 5-Gauss line (Fig. 1). Note that the probe housing is Resin, a non-magnetic, non-conductive polymer. Optical fibers (silica core, acrylate coating) exhibit negligible magnetic susceptibility (χ < 10⁻⁶), preventing image distortion. The complete material list is provided in Supplementary Table 1. This architecture enables artifact-free operation during active MRI scanning, as validated through simultaneous hemodynamic monitoring and gradient-echo imaging (see Results). The absence of radiofrequency (RF) coupling between the optical system and the MRI coil preserves both measurement fidelity and imaging quality, addressing a critical unmet need in MRI-guided interventions and functional imaging studies requiring continuous vital sign monitoring. System Characterization To validate the tissue penetration and vessel tracking capabilities of the fiber-optoacoustic system, we performed ex vivo experiments using a radial artery phantom (Fig. 2a). A soft silicone tube (diameter: 3 mm; wall thickness: 0.7 mm; Tygon S-50-HL, Saint-Gobain) was embedded in chicken breast tissue at a depth of 1.5 mm to approximate the typical position of the radial artery in adults. The tube was positioned parallel to the tissue surface directly beneath the probe. We evaluate ultrasonic performance—including emission capability and detection sensitivity—the lumen was alternately filled with water and air. The probe operated at a 500 Hz pulse repetition rate, with acoustic echoes recorded by the fiber laser sensor at a 500 MHz sampling rate (see Methods). As shown in Fig. 2a (bottom), M-mode signals were extracted for both conditions. In the water-filled phase, distinct posterior wall echoes confirmed effective acoustic transmission, attributed to the impedance matching between water (~1.48 MRayl) and the tissue phantom (~1.5 MRayl). Conversely, in the air-filled phase, the significant impedance mismatch between the anterior wall and the air core (~0.0004 MRayl) caused nearly total reflection at the interface, resulting in the complete absence of posterior echoes. Quantitative calibration (detailed in Supplementary Materials) demonstrates that the device generates a peak acoustic pressure of 1.5 MPa with a −6 dB working bandwidth of 20 MHz (see Fig. 2b, Incident US Signal). Although spectral analysis of the anterior wall echo (Fig. 2c, L1 Echo Signal) reveals a center frequency of 8 MHz and a −6 dB bandwidth of 10 MHz (reduced by high-frequency ultrasound attenuation and phase aberration in biological tissue), the acoustic signal retains sufficient spectral content for high-resolution measurement after tissue propagation. The echo pulse width indicates a spatial pulse length (SPL) of approximately 77 μm (FWHM). While SPL represents the fundamental axial resolution limit for distinguishing adjacent interfaces (e.g., differentiating anterior from posterior walls), it does not dictate the limit for dynamic displacement tracking. Under high-SNR conditions, the tracking precision is primarily constrained by the system's temporal discretization. With the implemented sampling rate of 500 MHz (2 ns time resolution) and an acoustic velocity of 1540 m/s, the system achieves an effective digital tracking spatial resolution of approximately 3 μm, enabling precise monitoring of subtle vascular wall dynamics [30]. In Vivo Arterial Diameter Tracking and waveform reconstruction Blood pressure is the force exerted by circulating blood against vessel walls per unit area. Its dynamic characteristics reflect the status of the cardiovascular system in real time, continuous BP monitoring is essential for the optimal diagnosis and treatment of many cardiovascular conditions. In this context, ultrasound-based measurement has demonstrated high accuracy. This method exploits the intrinsic coupling between arterial blood pressure and vessel diameter, governed by the arterial wall's elastic compliance (Fig. 3a). During systole, increased intraluminal pressure distends the artery, causing radial expansion that displaces the anterior and posterior walls in opposite directions. Conversely, during diastole, elastic recoil restores the vessel to its smaller diameter. By tracking the time-of-flight variations of ultrasound echoes reflecting from both walls, the instantaneous vessel diameter D(t), can be derived from the temporal difference between the anterior and posterior signals. This diameter is subsequently converted into blood pressure values using an exponential compliance model, calibrated against reference readings from a sphygmomanometer [30]. Raw M-mode ultrasound data (Fig. 3c, left) exhibit tissue echoes from skin, subcutaneous fat, and muscle layers, which obscure the arterial wall signals (signal-to-noise ratio (SNR) of 31 dB). This process converts ultrasound data to blood pressure through three key steps. (i) Signal enhancement: Frequency-domain deconvolution enhances arterial wall echoes while suppressing tissue clutter and noise. (ii) Signal extraction and Diameter calculation: Frame-by-frame peak detection extracts anterior/posterior wall positions to obtain pulsatile diameter changes (~2.1–2.3 mm range). (iii) Blood pressure conversion: The blood pressure measurement in this study is based on a physical model relating arterial diameter to blood pressure. The calibration procedure is carried out as follows: First, with the subject in a stable physiological state, systolic pressure ( ) and diastolic pressure ( ) are measured at the brachial artery using a standard sphygmomanometer, while the corresponding arterial cross-sectional areas at systole and diastole ( and ) are simultaneously recorded. Subsequently, the vascular rigidity coefficient , which reflects the individual’s vascular elasticity under the current physiological condition, is calculated using the formula . Once calibrated, the system only needs to continuously monitor the arterial diameter waveform and its minimal diastolic diameter to reconstruct the continuous blood pressure waveform . The reconstructed pressure waveform (Fig. 3b) preserves critical morphological features essential for comprehensive cardiovascular assessment: (i) Systolic Upstroke: The steep pressure rise (upstroke gradient) corresponds to left ventricular contractility and aortic valve patency. A blunted gradient may indicate impaired myocardial function or aortic stenosis [32]. (ii) Systolic Blood Pressure (SBP): The peak pressure recorded during ventricular ejection. (iii) Diastolic Blood Pressure (DBP): The nadir pressure observed during ventricular filling. (iv) Dicrotic Notch Time (DNT): Defined as the temporal interval between the systolic peak and the dicrotic notch, the DNT reflects the status of the arterial system at end-systole. High arterial compliance typically delays the notch (prolonging the DNT), whereas arterial stiffness accelerates wave reflection, causing the notch to appear earlier (shortening the DNT) [33, 34]. (v) Dicrotic Wave Index (DWI): Calculated as the ratio of the dicrotic wave amplitude to the systolic peak amplitude, this index serves as a standard for assessing arterial compliance and peripheral vascular resistance. Collectively, the preservation of the waveform contour facilitates pulse wave analysis beyond static systolic/diastolic values. These metrics are clinically significant for the early detection of hypertension, heart failure, and valvular pathologies [35-43], underscoring the potential of the FOAS device as a comprehensive wearable cardiovascular monitoring platform. We validated the FOAS in a high-field MRI environment (GE Discovery MR750, 3.0T) to highlight the distinct advantages of the "Ultrasound over Fiber" concept (Fig. 3d). The system's hybrid photoacoustic-acousto-optic interface enables the excitation and detection of ultrasound strictly via optical waveguides, supporting signal transmission over distances exceeding 10 meters. This capability allows for remote monitoring from outside the MRI suite, completely decoupling sensitive interrogation electronics from the scanner's electromagnetic interference. During in-vivo testing on a healthy volunteer (24-year-old male), the device demonstrated stable performance during both "scanner OFF" and "scanner ON" sessions. The complete absence of imaging artifacts and signal degradation confirms robust bidirectional electromagnetic compatibility. Thanks to its remote interrogation unit and metal-free sensor design, the FOAS successfully captured comprehensive hemodynamic data—ranging from fundamental metrics like blood pressure and heart rate to intricate waveform morphology such as the dicrotic notch—even during active scanning, extending the horizon for safe, continuous physiological monitoring in MRI settings. Multiscenario Hemodynamic Responses To demonstrate the ability of the wearable FOAS to capture rapid hemodynamic dynamics, we employed a occlusion-restitution test (see Methods). At s, a sphygmomanometer cuff was inflated to 150 mmHg—exceeding systolic pressure—and subsequently released gradually at s. The M-mode analysis (Fig. 4a) identifies three phases. First, during the initial inflation stage, the vascular pulse amplitude weakens, and the arterial diameter gradually expands (Fig. 4a iii). Second, when the cuff is fully inflated and pressure surpasses the systolic limit, the arterial diameter contracts and pulsation completely disappears (Fig. 4a iv). Finally, during the reperfusion transient, an abrupt diameter increase (~0.4 mm) occurs as pressure drops below systolic levels, marking the rapid reestablishment of flow and the restoration of pulsatility (Fig. 4a v). The observed vascular recovery speed is indicative of vessel elasticity and microcirculatory condition. Benefiting from high resolution and signal-to-noise ratio, this device goes beyond basic heart rate and blood pressure estimation to provide detailed pulse waveforms through direct vessel measurement. This enables precise cardiovascular health assessment during daily routine and monitoring of medication impacts. We present two typical tests here: In the exercise scenario (Fig. 4b), continuous monitoring immediately after 10 minutes of moderate-intensity cycling showed increases of 27 mmHg (SBP) and 18 mmHg (DBP), with average heart rate rising from 88 to 106 bpm. The waveform morphology shifted systematically: peripheral vasodilation reduced pulse wave reflection, leading to attenuated tidal and dicrotic components and a sharper single-peak morphology; the systolic-to-dicrotic interval DNT prolonged (0.2 s to 0.22 s, 10% increase), indicating reduced peripheral resistance and enhanced arterial compliance [44, 45]. These observations align with exercise-induced hemodynamic models and demonstrate the device’s sensitivity to rapid dynamics and waveform morphology. In the caffeine intake challenge (Fig. 4c), re-measurement 30 minutes after consuming 200 mL coffee revealed modest yet consistent BP elevations (SBP +6 mmHg; DBP +4 mmHg) with increased heart rate. In contrast to exercise, the waveform exhibited enhanced dicrotic components and a deepened notch, with shortened DNT (0.24 s to 0.16 s, 33% decrease), reflecting caffeine-induced vasoconstriction and increased pulse wave reflection due to higher peripheral resistance [46, 47]. Simultaneously, caffeine stimulation enhances myocardial contractility, thereby increasing stroke volume and ejection velocity [48]. The opposite trends of dicrotic amplitude and DNT across the pre/post caffeine consumption and pre/post exercise indicate that the device discriminates vasodilatory versus vasoconstrictive states, enabling physiologically grounded interpretation at beat-to-beat resolution. 5. Individual Variability in BP Waveform Morphology and Vascular Compliance Assessment To evaluate system repeatability and inter-subject discriminability, we performed continuous noninvasive monitoring on a cohort of 10 healthy volunteers (demographics in Supplementary Table 2). While all subjects maintained normotensive blood pressure, the extracted beat-to-beat waveforms revealed distinct inter-individual morphological heterogeneity, with the system consistently resolving fine features such as the dicrotic notch across all age groups (Fig. 5a). To decode these morphological variations, we analyzed the Dicrotic Notch Time (DNT)—a metric governed by left-ventricular ejection time, peripheral vascular resistance, and the timing of wave reflections [33, 34]. Comparison of DNT against key hemodynamic parameters revealed two significant population-level trends (Fig. 5b). First, DNT is negatively correlated with Diastolic Blood Pressure (DBP) (Fig. 5b-(1)). Physiologically, reduced peripheral resistance and higher vascular compliance delay the return of reflected waves (prolonging DNT) while facilitating diastolic runoff, thereby lowering DBP. Conversely, elevated resistance or stiffness accelerates pulse wave velocity and reflection strength, resulting in an earlier notch appearance (shortened DNT) and higher pressure. Second, DNT exhibits a strong negative correlation with Heart Rate (HR) (Fig. 5b-(2)). This aligns with the cardiac cycle dynamics, where higher heart rates shorten the ejection phase and accelerate aortic valve closure, effectively reducing the systolic-peak-to-notch interval. Although the Dicrotic Wave Index (DWI) displayed pronounced individual-specific variance (Fig. 5b-(3)), a discernible inverse relationship with age was observed. Specifically, DWI tended to decrease in older subjects, a phenomenon likely attributable to age-related arterial stiffening, which diminishes the vessel's elastic recoil capacity and consequently attenuates the dicrotic wave. Collectively, these morphological insights—alongside the robust correlations observed between DNT and hemodynamic parameters (DBP, HR)—confirm that the extracted waveform features serve as valuable biomarkers. These findings validate the FOAS's capability to distinguish subtle inter-subject differences in vascular compliance and peripheral resistance, consistent with arterial wave propagation theory. Discussion and Conclusion In this work, we achieved a fully integrated, electromagnetically neutral system by realizing a hybrid fiber-optoacoustic Interface, which synergizes wavefront-engineered photoacoustic generation with active fiber-laser detection. This work established "Ultrasound over Fiber" as a potential methodology for continuous hemodynamic monitoring, specifically engineered to bridge the gap between deep-tissue sensing and electromagnetic compatibility. This metal-free architecture not only ensures intrinsic immunity to electromagnetic interference (EMI), redefining physiological tracking in high-field MRI environments, but also fundamentally surpasses the limitations of conventional optical sensors (e.g., PPG) [ 14 – 17 ]. Unlike surface-limited photometric techniques, our system leverages ultrasonic M-mode to directly interrogate subcutaneous arterial mechanics [ 26 , 27 ]. By bypassing the viscoelastic filtering of skin and tissue, it captures precise vascular dimensional dynamics and reconstructs high-fidelity blood pressure waveforms that faithfully reflect central cardiovascular status. To achieve such high performance within a compact wearable footprint, we introduced a triad of innovations designed to circumvent the limitations of conventional all-optical ultrasound sensors: (1) A concave photoabsorptive interface allows for acoustic energy focusing, thereby maximizing opto-mechanical coupling efficiency; (2) Leveraging the high sensitivity of a fiber laser sensor, we attained a 31 dB in vivo SNR without compromising environmental stability; (3) A deconvolution-based processing pipeline suppresses clutter from non-vascular tissues to precisely isolate arterial wall signals. The system's robustness was confirmed through artifact-free operation within an active 3T MRI scanner, representing a paradigm shift in unobstructed vital sign monitoring for patients in high-magnetic-field settings. Looking forward, the translation of this all-optical modality into clinical workflows holds the promise of addressing the blind spot in current critical care [ 19 – 21 ]. Currently, MRI-guided interventions (e.g., cardiac catheterization, tumor ablation) often lack real-time hemodynamic feedback because standard electrical sensors introduce heating hazards or image artifacts. Our "electromagnetically transparent" technology may fill this void, potentially enabling a new method of intraoperative monitoring where patient stability is tracked continuously without interrupting imaging sequences. Furthermore, consistent with emerging trends in wearable electronics, the high-fidelity acoustic signatures captured by our device are ripe for AI-driven analytics. By coupling these rich optical-ultrasound datasets with deep learning algorithms, future systems could automate the extraction of latent digital biomarkers, such as stroke volume variability and vascular aging indices, effectively transforming the device from a passive monitor into an intelligent diagnostic companion for precision medicine. Despite these encouraging prospects, this study identifies directions for future evolution. First, absolute BP quantification still relies on initial cuff-based calibration. Future work will explore calibration-free models based on pulse wave velocity (PWV) or arterial wall elasticity to enhance system autonomy. Second, while the current single-point measurement is effective, extending this technology to sensor arrays could enable tomographic ultrasound imaging for enhanced cardiovascular profiling. Third, to extend monitoring from major arteries to microvasculature, future iterations will focus on broadening the spectral response by optimizing acoustic impedance matching and miniaturizing active elements to resolve higher-frequency components. Methods 1. Coating Structure and Materials The optoacoustic source employs a hybrid composite coating. The coating comprises two functional material [ 49 – 60 ] (Supplementary Fig. 1a): Absorption material: Carbon black provides high optical absorption and rapid photothermal conversion, effectively matching the laser pulse duration to maximize thermoelastic efficiency. Expansion material: Polydimethylsiloxane (PDMS, Sylgard 184, Dow Corning, 10:1 base-to-curing agent ratio) provides acoustic impedance matching to tissue (~ 1.5 MRayl vs. 1.4 MRayl for soft tissue. Upon nanosecond laser pulse irradiation (532 nm, 8 ns FWHM, 200 µJ/pulse, 500 Hz repetition rate), localized heating in the absorption layer induces rapid thermal expansion of the PDMS, launching acoustic waves into the surrounding medium. The mass-to-volume ratio of the two functional materials is formulated at 25 mg carbon powder per 0.1 mL PDMS. The blended composite is deposited as a uniform thin-film coating through a two-stage spin-coating protocol: an initial spreading phase at 900 rpm for 15 seconds ensures homogeneous distribution, followed by a thickness-control phase at 5000 rpm for 60 seconds. The coating was cured in a 100°C vacuum drying oven for 2 hours. This process yields a consolidated coating with thickness ranging from 90 to 100 µm. 2. Acoustic Focusing and Beam Shaping To concentrate acoustic energy at the radial artery depth and reduce clutter from surrounding tissues, a plano-concave acoustic mold is integrated into the probe housing. The mold is fabricated from transparent resin using 3D printing technology. The prepared optoacoustic coating is applied to the concave surface of the mold using UV-curable adhesive and subsequently solidified. After curing, the acoustic source is oriented with its concave surface facing the biological tissue. The 7 mm radius of curvature is designed based on the probe standoff distance (4 mm from the mold to the skin surface) and the target artery depth (3–5 mm beneath the skin), placing the acoustic focus at ~ 7 mm from the mold surface. Acoustic field mapping via calibrated needle hydrophone (0.2 mm aperture, Precision Acoustics) on a motorized XY stage (0.02 mm step size, 6 mm × 6 mm scan area) confirms a − 6 dB focal zone of 0.5 mm (lateral) × 1.4 mm (axial) at 7 mm depth, corresponding to a numerical aperture of 0.21 (Supplementary Fig. 1e). Frequency analysis of the acoustic waveform reveals a center frequency of 8 MHz with a − 6 dB bandwidth of 8–25 MHz, sufficient for arterial wall imaging with ~ 40 µm axial resolution (calculated as 0.5 × speed of sound / bandwidth = 0.5 × 1540 m/s / 20 MHz). Peak pressure at the focus reaches 1.5 MPa (measured at 200 µJ laser energy), representing a threefold enhancement compared to the unfocused configuration (0.5 MPa, Supplementary Fig. 1c, e). The focused beam also exhibits > 10 dB signal-to-noise ratio improvement at the center frequency, reducing interference from subcutaneous fat and muscle layers. 3. Reproducibility and Stability of the Optoacoustic Source To assess batch-to-batch consistency, five optoacoustic sources were fabricated using the optimized spin-coating protocol. Acoustic pressure measurements at 200 µJ laser energy yielded 1.48 ± 0.12 MPa (coefficient of variation 8%), confirming reproducible performance. Long-term stability was evaluated by operating a single source at maximum safe energy (200 µJ, corresponding to ~ 6 mJ/cm² fluence on the coating) for 30 minutes (9 × 10 5 pulses). The acoustic amplitude remained within 4000 ± 500 a.u. (12% variation), with no visible coating degradation or delamination (Supplementary Fig. 1d). 4. Safety Compliance The acoustic output complies with FDA guidelines for diagnostic ultrasound (FDA 510(k) Track 3). The mechanical index MI = peak negative pressure / √center frequency = 1.5 MPa / √15 MHz = 0.39 < 1.9 (regulatory limit). The spatial-peak pulse-average intensity I_SPPA = (peak pressure)² / (2 × acoustic impedance × pulse repetition frequency) = (1.5 MPa)² / (2 × 1.5 MRayl × 1 kHz) = 120 W/cm² < 190 W/cm² (regulatory limit). Thermal effects are negligible due to the low duty cycle (8 ns pulse width × 500 Hz = 4 × 10⁻⁶ duty cycle), resulting in < 0.1°C temperature rise at the skin surface (estimated via finite element thermal modeling). The human blood pressure monitoring experiments were conducted in accordance with a protocol approved by the Ethics Committee of Jinan University (Approval Number: JNUKY-2023-0151, n = 10 healthy volunteers). Total ten healthy adult male and female volunteers, aged from 22 to 37 years, participated in the study. Sex and gender were not taken into consideration in this experimental design and all participants were randomly selected. 5. Fiber Laser Ultrasound Sensor Acoustic echo detection employs a distributed Bragg reflector (DBR) fiber laser sensor embedded in an erbium-ytterbium codoped fiber core. The sensor architecture and operating principle are illustrated in Supplementary Fig. 2a. The laser cavity, contains two wavelength matched Bragg gratings (central wavelength: 1550 nm, > 25 dB in reflectivity), inscribed in erbium-ytterbium doped fiber (EYDF, absorption coefficient over 1000 dB/m at 1530 nm) using phase-mask UV lithography. The laser cavity length is about 3 mm, ensuring a single-longitudinal-mode operation. The cavity is pumped by a 980 nm laser diode via a wavelength-division multiplexer (WDM), generating single-frequency lasing at 1550 nm with a kHz-level linewidth. Incident acoustic waves induce strain in the fiber through photoelastic effect, modulating the effective refractive index and physical length of the optical fiber. The induced laser frequency variation was then readout via a delayed self-heterodyne method, which the sensor achieves an average noise-equivalent pressure density (NEP) of ~ 15 Pa over a bandwidth of 1–40 MHz. Note that laser oscillation effectively enhances acoustic responsivity by three orders of magnitude. The cylindrical fiber geometry (125 µm diameter) provides omnidirectional reception with a 180° acceptance angle in the plane perpendicular to the fiber axis, relaxing alignment constraints for wearable deployment. The fiber laser sensor enables seamless integration into a compact probe housing alongside the optoacoustic source. Acoustic sensitivity is calibrated using a broadband piezoelectric transducer (V214-BC-RM, Olympus-NDT, center frequency 50 MHz) in a water tank. The transducer generates plane waves of known pressure amplitude (measured via calibrated hydrophone), and the fiber sensor output is recorded interferometrically based on the IQ demodulation module described in the Supplementary Materials. 6. Phantom Fabrication and Acoustic Properties The chicken breast tissue phantom was prepared from fresh chicken breast (purchased from local market, stored at 4°C, used within 24 hours). The tissue was cut into 50 mm × 20 mm × 20 mm blocks, and a channel (3 mm diameter) was created at 2 mm depth using a biopsy punch. The silicone tube was inserted into the channel and secured with tissue adhesive. Acoustic properties of the chicken breast tissue were characterized via through-transmission measurements using a broadband transducer (V214-BB-RM, Olympus): speed of sound c = 1560 ± 20 m/s, attenuation coefficient α(f) = 1.15 * f dB/cm (with f in MHz), consistent with literature values for muscle tissue. 7. Blood Pressure Waveforms During Pressurization We performed continuous radial arterial monitoring on a healthy volunteer (male, 24 years, no history of cardiovascular disease) over 60 seconds. The device was positioned over the right radial artery with and secured via an elastic wristband, while a reference sphygmomanometer (Omron U724J) was placed on the right upper arm for pressurization (Fig. 4 a). The blood pressure cuff was inflated to 150 mmHg at t = 15 s and gradually deflated from t = 35 s. M-mode imaging revealed three characteristic phases: transient arterial expansion and reduced pulse amplitude during early inflation, suppressed pulsations under supra-systolic pressure, and a 0.4 mm step-change in diameter with restored flow upon deflation. 8. Signal Processing Raw M-mode ultrasound data (Fig. 3 c, left) exhibit tissue echoes from skin, subcutaneous fat, and muscle layers, which obscure the arterial wall signals. To enhance the signal-to-noise ratio and suppress non-arterial clutter, we applied frequency-domain deconvolution using the anterior wall echo as the point spread function (PSF). The PSF is windowed via a Hamming function to prevent spectral leakage, and the entire M-mode image is deconvolved in the Fourier domain. Spectral components with high correlation to the arterial wall's frequency signature are enhanced, while low-frequency tissue clutter ( 20 MHz) are suppressed. The deconvolved image (Fig. 3 c, bottom left) exhibits sharper, more concentrated arterial wall echoes, facilitating robust peak detection. Through peak detection extracts the depth positions of anterior and posterior walls frame-by-frame (Fig. 3 c, middle panels), and their difference yields the diameter time series (Fig. 3 c, right panel, 2.1–2.3 mm range with ~ 0.2 mm pulsatile amplitude). Using the calibrated compliance model: $$\:P\left(t\right)={P}_{d}\cdot\:{e}^{\alpha\:\left(\frac{D\left(t\right)}{{D}_{d}}-1\right)}\:\:\:$$ 1 with parameters \(\:{P}_{d}=68\:mmHg\:\) (diastolic pressure from sphygmomanometer), \(\:{D}_{d}=2.12mm\) (diastolic diameter), and \(\:\alpha\:\) (arterial stiffness coefficient), the diameter waveform is converted to the blood pressure waveform shown in Fig. 3 c, bottom right. Since the vascular rigidity coefficient \(\:\alpha\:\) and diastolic pressure \(\:{P}_{d}\) do not differ significantly across different arterial sites within the same subject, this one-time calibration parameter is applicable for subsequent blood pressure waveform reconstruction at other anatomical locations (such as the wrist, neck, and foot) without the need for frequent recalibration, unless the subject undergoes significant physiological or vascular structural changes. Declarations This study has been approved by the relevant institutional ethics committee (IRB of Jinan University, Approval No. JNUKY-2023-0151). All participants were informed of the study content and provided written informed consent prior to participation. Acknowledgements This study was supported by funding from Natural Science Foundation of China (no. 62322506 to Y.L., no. 62275104 to Y.L., no. 62135006 to B.-O. G.), National Key Technologies R&D Program of China (no. 2023YFF0715302 to L.J.), Research Teams Project of Guangdong Pearl River Talents Program (no. 2019BT02X105 to B.-O. G.), Science and Technology Projects in Guangzhou (no. 2024B03J0254 to L.J.). Noncommunicable Chronic Diseases-National Science and Technology Maior Project (2023ZD0505500). Conflict of Interest The authors declare no conflict of interest. References A. Bergholz, G. Greiwe, K. Kouz, and B. Saugel, "Continuous Blood Pressure Monitoring in Patients Having Surgery: A Narrative Review," (in eng), Medicina (Kaunas), vol. 59, no. 7, Jul 14 2023, doi: 10.3390/medicina59071299. B. H. McGhee and E. J. Bridges, "Monitoring arterial blood pressure: what you may not know," (in eng), Crit Care Nurse, vol. 22, no. 2, pp. 60-4, 66-70, 73 passim, Apr 2002. G. J. Langewouters, J. J. Settels, R. Roelandt, and K. H. Wesseling, "Why use Finapres or Portapres rather than intra-arterial or intermittent non-invasive techniques of blood pressure measurement?," (in eng), J Med Eng Technol, vol. 22, no. 1, pp. 37-43, Jan-Feb 1998, doi: 10.3109/03091909809009997. A. S. Meidert and B. Saugel, "Techniques for Non-Invasive Monitoring of Arterial Blood Pressure," (in eng), Front Med (Lausanne), vol. 4, p. 231, 2017, doi: 10.3389/fmed.2017.00231. L. Geddes, M. Voelz, C. Combs, D. Reiner, and C. F. Babbs, "Characterization of the oscillometric method for measuring indirect blood pressure," Annals of biomedical engineering, vol. 10, no. 6, pp. 271-280, 1982. M. Lin et al. , "A fully integrated wearable ultrasound system to monitor deep tissues in moving subjects," Nature biotechnology, vol. 42, no. 3, pp. 448-457, 2024. J.-É. S. Kenny, "Functional hemodynamic monitoring with a wireless ultrasound patch," Journal of Cardiothoracic and Vascular Anesthesia, vol. 35, no. 5, pp. 1509-1515, 2021. J. R. Sempionatto et al. , "An epidermal patch for the simultaneous monitoring of haemodynamic and metabolic biomarkers," Nature Biomedical Engineering, vol. 5, no. 7, pp. 737-748, 2021. J. Li et al. , "Recent Advances in Materials, Devices and Algorithms Toward Wearable Continuous Blood Pressure Monitoring," ACS Nano, vol. 18, no. 27, pp. 17407-17438, 2024, doi: 10.1021/acsnano.4c04291. J. Guo et al. , "Fiber Cladding Waveguide Phase-Shifted FBG Encapsulated in Flexible Material for Estimating Human Blood Pressure," Journal of Lightwave Technology, vol. 43, no. 7, pp. 3261-3268, 2025, doi: 10.1109/jlt.2024.3515043. D. Jia et al. , "A Fiber Bragg Grating Sensor for Radial Artery Pulse Waveform Measurement," IEEE Transactions on Biomedical Engineering, vol. 65, no. 4, pp. 839-846, 2018, doi: 10.1109/tbme.2017.2722008. Y. Haseda, J. Bonefacino, H.-Y. Tam, S. Chino, S. Koyama, and H. Ishizawa, "Measurement of pulse wave signals and blood pressure by a plastic optical fiber FBG sensor," Sensors, vol. 19, no. 23, p. 5088, 2019. Z. Tang, S. Wang, and C. Shi, "Development of a hybrid force-displacement sensor based on fiber Bragg grating for radial artery pulse waveform measurement," IEEE sensors journal, vol. 21, no. 18, pp. 20045-20054, 2021. S. Haddad, A. Boukhayma, and A. Caizzone, "Continuous PPG-Based Blood Pressure Monitoring Using Multi-Linear Regression," (in eng), IEEE J Biomed Health Inform, vol. 26, no. 5, pp. 2096-2105, May 2022, doi: 10.1109/jbhi.2021.3128229. G. Chen, L. Zou, and Z. Ji, "A review: Blood pressure monitoring based on PPG and circadian rhythm," (in eng), APL Bioeng, vol. 8, no. 3, p. 031501, Sep 2024, doi: 10.1063/5.0206980. P. H. Charlton et al. , "Assessing hemodynamics from the photoplethysmogram to gain insights into vascular age: a review from VascAgeNet," American Journal of Physiology-Heart and Circulatory Physiology, vol. 322, no. 4, pp. H493-H522, 2022. G. Chan et al. , "Multi-site photoplethysmography technology for blood pressure assessment: challenges and recommendations," Journal of clinical medicine, vol. 8, no. 11, p. 1827, 2019. R. Dagenais and G. D. Mitsis, "Non-invasive estimation of arterial blood pressure fluctuations using a peripheral photoplethysmograph inside the MRI scanner," (in eng), Annu Int Conf IEEE Eng Med Biol Soc, vol. 2023, pp. 1-4, Jul 2023, doi: 10.1109/embc40787.2023.10340020. R. J. Colchester et al. , "All-Optical Rotational Ultrasound Imaging," (in eng), Sci Rep, vol. 9, no. 1, p. 5576, Apr 3 2019, doi: 10.1038/s41598-019-41970-z. K. Pham et al. , "Broadband All-Optical Plane-Wave Ultrasound Imaging System Based on a Fabry-Perot Scanner," (in eng), IEEE Trans Ultrason Ferroelectr Freq Control, vol. 68, no. 4, pp. 1007-1016, Apr 2021, doi: 10.1109/tuffc.2020.3028749. E. Z. Zhang and P. C. Beard, "A miniature all-optical photoacoustic imaging probe," in Photons plus ultrasound: imaging and sensing 2011 , 2011, vol. 7899: SPIE, pp. 291-296. J. E. Sharman et al. , "Validation of non-invasive central blood pressure devices: ARTERY Society task force consensus statement on protocol standardization," European Heart Journal, vol. 38, no. 37, pp. 2805-2812, 2017, doi: 10.1093/eurheartj/ehw632. P. Salvi et al. , "Non-invasive assessment of arterial stiffness: pulse wave velocity, pulse wave analysis and carotid cross-sectional distensibility: comparison between methods," Journal of Clinical Medicine, vol. 11, no. 8, p. 2225, 2022. G. Kober and J. O. Arndt, "[Pressure-diameter relationship in the common carotid artery of conscious man]," (in ger), Pflugers Arch, vol. 314, no. 1, pp. 27-39, 1970, doi: 10.1007/bf00587044. Die Druck-Durchmesser-Beziehung der A. carotis communis des wachen Menschen. J. O. Arndt and G. Kober, "[Pressure-diameter relationship of the intact femoral artery in conscious man, and its influencing by noradrenaline infusions]," (in ger), Pflugers Arch, vol. 318, no. 2, pp. 130-46, 1970, doi: 10.1007/bf00586492. Die Druck-Durchmesser-Beziehung der intakten A. femoralis des wachen Menschen und ihre Beeinflussung durch Noradrenalin-Infusionen. X. Gao et al. , "A wearable echomyography system based on a single transducer," Nature Electronics, vol. 7, no. 11, pp. 1035-1046, 2024, doi: 10.1038/s41928-024-01271-4. M. Lin et al. , "A fully integrated wearable ultrasound system to monitor deep tissues in moving subjects," Nature Biotechnology, vol. 42, no. 3, pp. 448-457, 2023, doi: 10.1038/s41587-023-01800-0. E. Svanström, T. Linder, and T. Löfqvist, "Analytical one-dimensional model for laser-induced ultrasound in planar optically absorbing layer," (in eng), Ultrasonics, vol. 54, no. 3, pp. 888-93, Mar 2014, doi: 10.1016/j.ultras.2013.10.019. R. K. Poduval et al. , "Optical fiber ultrasound transmitter with electrospun carbon nanotube-polymer composite," (in eng), Appl Phys Lett, vol. 110, no. 22, p. 223701, May 29 2017, doi: 10.1063/1.4984838. C. Wang et al. , "Monitoring of the central blood pressure waveform via a conformal ultrasonic device," Nature Biomedical Engineering, vol. 2, no. 9, pp. 687-695, 2018, doi: 10.1038/s41551-018-0287-x. S. Zhou et al. , "Clinical validation of a wearable ultrasound sensor of blood pressure," Nature biomedical engineering, vol. 9, no. 6, pp. 865-881, 2025. J. Manolas, P. Wirz, and W. Rutishauser, "Relationship between duration of systolic upstroke of apexcardiogram and internal indexes of myocardial function in man," American Heart Journal, vol. 91, no. 6, pp. 726-734, 1976. A. S. Alhakak, J. R. Teerlink, J. Lindenfeld, M. Böhm, G. M. Rosano, and T. Biering-Sørensen, "The significance of left ventricular ejection time in heart failure with reduced ejection fraction," European journal of heart failure, vol. 23, no. 4, pp. 541-551, 2021. T. Biering‐Sørensen et al. , "Left ventricular ejection time is an independent predictor of incident heart failure in a community‐based cohort," European journal of heart failure, vol. 20, no. 7, pp. 1106-1114, 2018. Z. Song et al. , "Bioinspired microstructured pressure sensor based on a janus graphene film for monitoring vital signs and cardiovascular assessment," Advanced Electronic Materials, vol. 4, no. 11, p. 1800252, 2018. Y. Liu, F. Meng, Y. Zhou, S. M. Mugo, and Q. Zhang, "Graphene oxide films prepared using gelatin nanofibers as wearable sensors for monitoring cardiovascular health," Advanced Materials Technologies, vol. 4, no. 11, p. 1900540, 2019. J. Wang et al. , "Diaphragm‐based optical fiber sensor for pulse wave monitoring and cardiovascular diseases diagnosis," Journal of Biophotonics, vol. 12, no. 10, 2019, doi: 10.1002/jbio.201900084. L. Li et al. , "Automatic and continuous blood pressure monitoring via an optical-fiber-sensor-assisted smartwatch," PhotoniX, vol. 4, no. 1, 2023, doi: 10.1186/s43074-023-00099-z. U. Pingali, C. Nutalapati, and S. Gundagani, "Effect of Omega‐3 Fatty Acid Alone and in Combination with Proprietary Chromium Complex on Endothelial Function in Subjects with Metabolic Syndrome: A Randomized, Double‐Blind, Parallel‐Group Clinical Study," Evidence‐Based Complementary and Alternative Medicine, vol. 2021, no. 1, p. 2972610, 2021. A. P. Avolio, M. Butlin, and A. Walsh, "Arterial blood pressure measurement and pulse wave analysis—their role in enhancing cardiovascular assessment," Physiological measurement, vol. 31, no. 1, p. R1, 2009. L. Xu, M. Q.-H. Meng, C. Shi, K. Wang, and N. Li, "Quantitative analyses of pulse images in Traditional Chinese Medicine," Medical Acupuncture, vol. 20, no. 3, pp. 175-189, 2008. D. Chambers, C. Huang, and G. Matthews, Basic physiology for anaesthetists . Cambridge University Press, 2019. E. Agabiti-Rosei et al. , "Central blood pressure measurements and antihypertensive therapy: a consensus document," Hypertension, vol. 50, no. 1, pp. 154-160, 2007. M. Horiuchi and K. Okita, "Blood flow restricted exercise and vascular function," International journal of vascular medicine, vol. 2012, no. 1, p. 543218, 2012. R. Q. Landers-Ramos, T. Silva, D. A. Dobrosielski, and N. D. Knuth, "Examination of post-exercise microvascular reactivity in healthy adults," European Journal of Applied Physiology, pp. 1-12, 2025. S. A. Rebello and R. M. van Dam, "Coffee consumption and cardiovascular health: getting to the heart of the matter," Current cardiology reports, vol. 15, no. 10, p. 403, 2013. A. Mahmud and J. Feely, "Acute effect of caffeine on arterial stiffness and aortic pressure waveform," Hypertension, vol. 38, no. 2, pp. 227-231, 2001. S. O'neill and D. Eisner, "A mechanism for the effects of caffeine on Ca2+ release during diastole and systole in isolated rat ventricular myocytes," The Journal of physiology, vol. 430, no. 1, pp. 519-536, 1990. S. Noimark et al. , "Carbon‐nanotube–PDMS composite coatings on optical fibers for all‐optical ultrasound imaging," Advanced Functional Materials, vol. 26, no. 46, pp. 8390-8396, 2016. B.-Y. Hsieh, J. Kim, J. Zhu, S. Li, X. Zhang, and X. Jiang, "A laser ultrasound transducer using carbon nanofibers–polydimethylsiloxane composite thin film," Applied Physics Letters, vol. 106, no. 2, 2015. S. H. Lee, Y. Lee, and J. J. Yoh, "Reduced graphene oxide coated polydimethylsiloxane film as an optoacoustic transmitter for high pressure and high frequency ultrasound generation," Applied Physics Letters, vol. 106, no. 8, 2015. Y. Li, Z. Guo, G. Li, and S.-L. Chen, "Miniature fiber-optic high-intensity focused ultrasound device using a candle soot nanoparticles-polydimethylsiloxane composites-coated photoacoustic lens," Optics express, vol. 26, no. 17, pp. 21700-21711, 2018. S. Noimark et al. , "Polydimethylsiloxane composites for optical ultrasound generation and multimodality imaging," Advanced Functional Materials, vol. 28, no. 9, p. 1704919, 2018. S. Berber, Y.-K. Kwon, and D. Tománek, "Unusually high thermal conductivity of carbon nanotubes," Physical review letters, vol. 84, no. 20, p. 4613, 2000. T. Ramanathan, F. Fisher, R. Ruoff, and L. C. Brinson, "Amino-functionalized carbon nanotubes for binding to polymers and biological systems," Chemistry of Materials, vol. 17, no. 6, pp. 1290-1295, 2005. P.-C. Ma, N. A. Siddiqui, G. Marom, and J.-K. Kim, "Dispersion and functionalization of carbon nanotubes for polymer-based nanocomposites: A review," Composites Part A: Applied Science and Manufacturing, vol. 41, no. 10, pp. 1345-1367, 2010. H. W. Baac et al. , "Carbon-nanotube optoacoustic lens for focused ultrasound generation and high-precision targeted therapy," Scientific reports, vol. 2, no. 1, p. 989, 2012. J. Kim, H. Kim, W.-Y. Chang, W. Huang, X. Jiang, and P. A. Dayton, "Candle-soot carbon nanoparticles in photoacoustics: advantages and challenges for laser ultrasound transmitters," IEEE nanotechnology magazine, vol. 13, no. 3, pp. 13-28, 2019. G. K. Sharma and N. R. James, "Carbon black incorporated carbon nanofiber based polydimethylsiloxane composite for electromagnetic interference shielding," Carbon Trends, vol. 8, p. 100177, 2022. M. S. Kang, H. Lee, S. J. Jeong, T. J. Eom, J. Kim, and D.-W. Han, "State of the art in carbon nanomaterials for photoacoustic imaging," Biomedicines, vol. 10, no. 6, p. 1374, 2022. Additional Declarations The authors declare no competing interests. Supplementary Files SupplementaryMaterial.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8635823","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":576572187,"identity":"0c12ed0e-e1a9-49ad-bb75-2a2b8aee032e","order_by":0,"name":"Zhixuan Hu","email":"","orcid":"https://orcid.org/0009-0006-2455-8246","institution":"College of Physics and Optoelectronic Engineering, Jinan University","correspondingAuthor":false,"prefix":"","firstName":"Zhixuan","middleName":"","lastName":"Hu","suffix":""},{"id":576572188,"identity":"457b2494-06d8-4664-9cc9-55e200b04c8c","order_by":1,"name":"Minghao Ma","email":"","orcid":"","institution":"School of Optoelectronic Science and Engineering, South China Normal University","correspondingAuthor":false,"prefix":"","firstName":"Minghao","middleName":"","lastName":"Ma","suffix":""},{"id":576572189,"identity":"84c46b0c-6acd-4995-8182-282e1710c7ee","order_by":2,"name":"Chaoneng Wu","email":"","orcid":"","institution":"College of Physics and Optoelectronic Engineering, Jinan University","correspondingAuthor":false,"prefix":"","firstName":"Chaoneng","middleName":"","lastName":"Wu","suffix":""},{"id":576572190,"identity":"b23c3a2d-d15a-43dc-aa38-b9f4fcab4e97","order_by":3,"name":"Xue Bai","email":"","orcid":"","institution":"College of Physics and Optoelectronic Engineering, Jinan University","correspondingAuthor":false,"prefix":"","firstName":"Xue","middleName":"","lastName":"Bai","suffix":""},{"id":576572191,"identity":"26cfb42d-6197-41e4-af9d-2187d6a9fd84","order_by":4,"name":"Wei Li","email":"","orcid":"","institution":"College of Physics and Optoelectronic Engineering, Jinan University","correspondingAuthor":false,"prefix":"","firstName":"Wei","middleName":"","lastName":"Li","suffix":""},{"id":576572192,"identity":"42a43a60-d15c-478f-8d0d-17e681266dcf","order_by":5,"name":"Zhongyuan Cheng","email":"","orcid":"","institution":"Department of Radiology, the First Affiliated Hospital, Jinan University","correspondingAuthor":false,"prefix":"","firstName":"Zhongyuan","middleName":"","lastName":"Cheng","suffix":""},{"id":576572193,"identity":"e8300bc4-501f-447e-bba3-a61cad96216f","order_by":6,"name":"Cong Mai","email":"","orcid":"","institution":"Department of critical care medicine. Guangdong Provincial People's Hospital","correspondingAuthor":false,"prefix":"","firstName":"Cong","middleName":"","lastName":"Mai","suffix":""},{"id":576572194,"identity":"50c1ed06-f929-4d30-ae82-6cc9642de712","order_by":7,"name":"Yizhi Liang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA3ElEQVRIiWNgGAWjYBACPiA+/KPCRo6xAcRlI0ILUA3jY4YzacYkaWE2Zmw7nNgA4xLWIpH+TLqALS29eXaPAcOHssMM/LMbCGlJSJOewWOT2zjnjAHjjHOHGSTuHCCo5ZgEj0RabuOMHANm3rbDDAYSCYS0JLZJ8BgcTmcEaflLnJZkZmOehMMJYC2MRGnhecb4cMaBNMPGGWkFB3vOpfNI3CCghZ89/cGBj/9s5A1nJG988KPMWo5/BgEtDAJQBYYNDAwHgDQPAfUgaw5AaHnCSkfBKBgFo2CkAgDPokEkdbVF/wAAAABJRU5ErkJggg==","orcid":"","institution":"College of Physics and Optoelectronic Engineering, Jinan University","correspondingAuthor":true,"prefix":"","firstName":"Yizhi","middleName":"","lastName":"Liang","suffix":""},{"id":576572195,"identity":"6ba94006-ffea-4ee2-8a8f-ca071cc763a5","order_by":8,"name":"Long Jin","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA2klEQVRIiWNgGAWjYBACxmYGxgNAWg4hdICwFrAaY+K1wNQkNhCthbmd98CBjztq09c2sD/8XNjGIMd3I4HxcwFeh/ElHJx55njutgM8xtIz2xiMJW8kMEvPwKuFx+Awb9sxkBY2Zt42hsQNNxLYmHmI0JJudoD9GUhLPbFaahLMDjCYgbQkGBCj5eDMtgOG2w4D/cJzTsJw5pmHzdL4tBj2nzF88LGtTt7sePvDzzxlNvJ8x5MPfsarpQFMHQYGN5ghAbK5AY8GBgZ5CFWHV9EoGAWjYBSMcAAA2uZKLB+obK8AAAAASUVORK5CYII=","orcid":"","institution":"School of Optoelectronic Science and Engineering, South China Normal University","correspondingAuthor":true,"prefix":"","firstName":"Long","middleName":"","lastName":"Jin","suffix":""},{"id":576572196,"identity":"9f731178-0ca6-46e4-aa26-965c244b3c98","order_by":9,"name":"Bai-ou Guan","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAy0lEQVRIiWNgGAWjYLACxgYGOSAGAjYiVPNAtRiTriURrIMoLfb8aww/F+6wSW+e3WPA8KHsMAP/7AYCtki8MZaeeSYtt3HOGQPGGecOM0jcOUBIyxkDad62w7mNM3IMmIEMBgOJBIJajH8DVaYzgrT8JUoLf48ZyJYEsBZGorTcYCuz5j2TZtg4I63gYM+5dB6JGwS0sPcf3nybd4eNvOGM5I0PfpRZy/HPIKCFAeYMwwYGhgMM0IjCD/gPQGh5wkpHwSgYBaNgpAIAKqRAYC18W3MAAAAASUVORK5CYII=","orcid":"","institution":"College of Physics and Optoelectronic Engineering, Jinan University","correspondingAuthor":true,"prefix":"","firstName":"Bai-ou","middleName":"","lastName":"Guan","suffix":""}],"badges":[],"createdAt":"2026-01-19 07:54:14","currentVersionCode":1,"declarations":{"humanSubjects":true,"vertebrateSubjects":false,"conflictsOfInterestStatement":false,"humanSubjectEthicalGuidelines":true,"humanSubjectConsent":true,"humanSubjectClinicalTrial":false,"humanSubjectCaseReport":false,"vertebrateSubjectEthicalGuidelines":false},"doi":"10.21203/rs.3.rs-8635823/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8635823/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":100951936,"identity":"957ae0ba-404f-432d-a425-bcffd2950e01","added_by":"auto","created_at":"2026-01-23 07:11:33","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":35388643,"visible":true,"origin":"","legend":"","description":"","filename":"HuManuscript.docx","url":"https://assets-eu.researchsquare.com/files/rs-8635823/v1/c0ae47610f0d068e26a7b259.docx"},{"id":100947035,"identity":"a853b6c2-52ab-4087-9855-90fcf6a9b985","added_by":"auto","created_at":"2026-01-23 06:26:39","extension":"json","order_by":1,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":342,"visible":true,"origin":"","legend":"","description":"","filename":"rs8635823.json","url":"https://assets-eu.researchsquare.com/files/rs-8635823/v1/78098cea4b22d2d911a1f4bb.json"},{"id":100947053,"identity":"74f86489-e48f-41ea-8a03-15d5d66c35c1","added_by":"auto","created_at":"2026-01-23 06:26:44","extension":"xml","order_by":2,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":131967,"visible":true,"origin":"","legend":"","description":"","filename":"rs86358230enriched.xml","url":"https://assets-eu.researchsquare.com/files/rs-8635823/v1/4a7580547a4922503727a4d7.xml"},{"id":100947048,"identity":"c8d4fef8-c8d9-4bb1-bd49-4a666efde6e3","added_by":"auto","created_at":"2026-01-23 06:26:43","extension":"png","order_by":3,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":16668593,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8635823/v1/535e8e2b9dfbf828fd374219.png"},{"id":100951780,"identity":"ea268689-ee3b-4214-93f2-64bae78c7560","added_by":"auto","created_at":"2026-01-23 07:11:14","extension":"png","order_by":4,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":5303509,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8635823/v1/bd19f5bac64afa5a307e7289.png"},{"id":100951062,"identity":"cd16a588-23d2-4e54-a134-a8854a27e7a4","added_by":"auto","created_at":"2026-01-23 07:09:56","extension":"png","order_by":5,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":4219243,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8635823/v1/807b2aef05e2118317e618fb.png"},{"id":100952098,"identity":"bac0da6d-64da-4337-b414-c2abc3b23a05","added_by":"auto","created_at":"2026-01-23 07:11:54","extension":"png","order_by":6,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":8472913,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8635823/v1/4942093e7a776507e63f7580.png"},{"id":100951910,"identity":"bfbe7d36-a381-4fd0-9c20-6f2344fab785","added_by":"auto","created_at":"2026-01-23 07:11:25","extension":"png","order_by":7,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":612529,"visible":true,"origin":"","legend":"","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8635823/v1/6a2c4ca8bee4f940abf03981.png"},{"id":100951262,"identity":"251561ae-be9c-41a0-9180-f719c33f15f3","added_by":"auto","created_at":"2026-01-23 07:10:21","extension":"png","order_by":8,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":1092687,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8635823/v1/6ac4986d03d9de6c7cacdad4.png"},{"id":100951079,"identity":"0a2a4cb2-3472-416f-b0e9-ebbde73cdc89","added_by":"auto","created_at":"2026-01-23 07:09:57","extension":"png","order_by":9,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":376249,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8635823/v1/1595b663279c42ef02b70f89.png"},{"id":100952108,"identity":"c55df1f2-e5cd-46bd-a086-2022d7450139","added_by":"auto","created_at":"2026-01-23 07:11:56","extension":"png","order_by":10,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":566095,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8635823/v1/c0ff5354169ca1facb7b6aa2.png"},{"id":100952255,"identity":"de4ff09a-ad95-43ed-9aff-6b40b82aed88","added_by":"auto","created_at":"2026-01-23 07:12:22","extension":"png","order_by":11,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":1188370,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8635823/v1/43d59be9e84b53d54193fe6e.png"},{"id":100947049,"identity":"be4100cb-c2a2-4087-afc1-9ec4cd3efc85","added_by":"auto","created_at":"2026-01-23 06:26:43","extension":"png","order_by":12,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":239788,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8635823/v1/d23b60ba5c2b912a49b4717a.png"},{"id":100952156,"identity":"19cc637e-8808-463a-8dc2-407ef26a5027","added_by":"auto","created_at":"2026-01-23 07:12:05","extension":"xml","order_by":13,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":129690,"visible":true,"origin":"","legend":"","description":"","filename":"rs86358230structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-8635823/v1/cd2da13bb3558e33ee4bb70a.xml"},{"id":100951931,"identity":"4dd9e196-d466-4bd2-92a5-19492977170a","added_by":"auto","created_at":"2026-01-23 07:11:33","extension":"html","order_by":14,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":145273,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8635823/v1/2a567bf6093b1cc4bea3b4e4.html"},{"id":100952100,"identity":"7728edc3-a825-4c99-a82e-b6eb1185a4cc","added_by":"auto","created_at":"2026-01-23 07:11:54","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":16668593,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFiber-optoacoustic system for MRI-compatible continuous arterial monitoring. \u003c/strong\u003e(a) Schematic of wearable operation inside an active MRI scanner. Inset : compact probe and illustration of vessel tracking principle, where pulsed laser excitation generates acoustic waves that propagate through tissue and reflect from arterial walls for diameter tracking. (b) System schematic illustrating the all-optical ultrasound interface: ultrasound excitation via pulsed laser delivery, acoustic echo reception by the fiber sensor with FPGA-based signal processing and data acquisition. The blood pressure waveform is obtained by sequentially applying deconvolution, vascular signal extraction, and blood pressure conversion to the original signal.\u003c/p\u003e\n\u003cp\u003e(AOM: Acousto-Optic Modulator; PD: Photodetector; DAQ: Data Acquisition; FPGA: Field-Programmable Gate Array.)\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8635823/v1/3c6d04098996b076ea5e33c6.png"},{"id":100952447,"identity":"112ad672-2025-45a4-9727-ea71bcd75234","added_by":"auto","created_at":"2026-01-23 07:16:28","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":5303509,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTissue penetration and acoustic characterization.\u003c/strong\u003e\u003cbr\u003e\n(a) In vitro validation using a vascular phantom (chicken breast with embedded soft tube). Left/Right: A-line signals acquired under water-filled and air-filled conditions. The water-filled state clearly resolves four interfaces: the anterior wall (L1, L2) and posterior wall (L3, L4), confirming effective penetrability (\u0026gt;5 mm) and clear structural delineation (3 mm diameter, 0.7 mm wall). In contrast, the air-filled state yields only anterior echoes due to total reflection at the lumen interface. (b) Calibrated acoustic output showing a peak pressure of 1.5 MPa at 7 mm distance (center frequency: 8 MHz; −6 dB bandwidth: 20 MHz). (c) Spectral analysis of the anterior echo (center frequency: 8 MHz; −6 dB bandwidth: 10 MHz), verifying the signal integrity required for high-precision displacement detection.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8635823/v1/d872ecb07657b5ead6f640d1.png"},{"id":100947047,"identity":"c896afc3-dd74-48af-b86e-763c697b7657","added_by":"auto","created_at":"2026-01-23 06:26:42","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":4219243,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eIn vivo arterial tracking and hemodynamic reconstruction.\u003c/strong\u003e\u003cbr\u003e\n(a) Principle of diameter-to-pressure conversion. Temporal synchronization between arterial wall motion and pressure waves, characterized by the anti-phase displacement of anterior and posterior walls. (b) Reconstructed pressure waveform annotated with morphological indices: SBP, DBP, upstroke gradient, DNT, and DWI. The high-resolution contour enables detailed cardiovascular assessment. (c) Signal processing workflow. Raw M-mode data are deconvolved to suppress noise, followed by automated tracking of diameter oscillations. The diameter time-series is mapped to pressure using a calibrated compliance model. (d) MRI compatibility demonstration. Continuous BP and heart rate monitoring with the MRI scanner \u003cstrong\u003eOFF (top)\u003c/strong\u003e and \u003cstrong\u003eON (bottom)\u003c/strong\u003e, demonstrating stable performance and immunity to electromagnetic interference during active imaging sequences.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8635823/v1/f3efb2c2f3d89246d6b49753.png"},{"id":100950945,"identity":"65310ea6-0877-475d-88b7-f1c6b5fe6815","added_by":"auto","created_at":"2026-01-23 07:09:40","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":8472913,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eContinuous hemodynamic monitoring under physiological challenges.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e(a) i: photos of occlusion-restitution test; ii: The three phases of cuff Inflation-deflation Cycle (three distinctly colored boxes, 15-45 s, detail shows in iii to v) demonstrates system response. (b) Pre- and post-exercise measurements showing elevated blood pressure, increased HR (****p£ 0.0001), weakened dicrotic wave amplitude, decreased DWI (****p £0.0001), and prolonged DNT (*p £0.05) following 10-minute moderate-intensity cycling. Normalized pulse waveforms highlighting exercise-induced morphological changes. (c) Pre- and post-caffeine consumption measurements demonstrating transient blood pressure elevation, enhanced dicrotic wave amplitude, increased HR (**p £0.01), shortened DNT (****p £0.0001) and increased DWI (****p £0.0001) due to caffeine-induced vasoconstriction. Normalized waveforms showing deepened dicrotic notch post-consumption.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8635823/v1/167626ef7f0c57a9393cf2e2.png"},{"id":100947050,"identity":"c9fdeffd-93f3-465d-ab82-fad839aab58b","added_by":"auto","created_at":"2026-01-23 06:26:43","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":612529,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eHemodynamic waveform variability and morphological correlations across subjects.\u003c/strong\u003e(a) Representative blood pressure waveforms from ten healthy subjects (S1-S10). (b) Statistical analysis of the correlations among DBP, HR, DWI, and DNT across all subjects. Correlation analysis between heart rate, diastolic blood pressure, and DNT reveals significant negative correlations between DNT and DBP as well as between DNT and HR, consistent with established principles of arterial hemodynamics.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8635823/v1/203b1cd03621495bdaabeb06.png"},{"id":100954208,"identity":"4bf16429-6d1e-41bb-9ff4-bf049f8cf3b6","added_by":"auto","created_at":"2026-01-23 07:24:02","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":34482824,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8635823/v1/fa51c326-9289-439d-97d8-72852f76fe45.pdf"},{"id":100950969,"identity":"255798b7-c23a-4dfe-b25b-367441501542","added_by":"auto","created_at":"2026-01-23 07:09:43","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":852504,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryMaterial.docx","url":"https://assets-eu.researchsquare.com/files/rs-8635823/v1/a8012bd83d0f9dd40c805cc9.docx"}],"financialInterests":"The authors declare no competing interests.","formattedTitle":"\u003cp\u003e\u003cstrong\u003eA Wearable Fiber-Optoacoustic Interface for Continuous Deep-Tissue Hemodynamics in MRI Environments\u003c/strong\u003e\u003c/p\u003e","fulltext":[{"header":"Introduction","content":"\u003cp\u003eContinuous hemodynamic monitoring--encompassing pulse waveform morphology, beat-to-beat blood pressure (BP) dynamics, and heart rate variability--is a cornerstone of intraoperative management and critical care [1, 2]. Currently, the clinical \"gold standard\" relies on invasive arterial catheterization, which couples intravascular pressure directly to an external transducer via a fluid-filled column. While offering precision measurement, this invasive approach carries inherent risks of infection, thrombosis, and nerve injury, strictly limiting its use to high-risk surgical patients. For the broader population, non-invasive alternatives are employed but face significant limitations. Oscillometric cuffs, though ubiquitous, provide only intermittent snapshots of cardiovascular status, missing rapid hemodynamic fluctuations [3-5]. Emerging wearable ultrasound patches have recently achieved deep-tissue imaging capabilities by utilizing piezoelectric arrays to transmit and receive acoustic waves [6-9]; however, their fundamental reliance on metallic interconnects and conductive circuits renders them strictly incompatible with Magnetic Resonance Imaging (MRI). Standard ultrasound probes rely on piezoelectric arrays and conductive interconnects, presenting significant hazards in MRI suites due to magnetic attraction forces and radiofrequency (RF) induced heating. Furthermore, these devices introduce susceptibility artifacts that degrade MRI image quality.\u003c/p\u003e\n\u003cp\u003eTo address this challenge, optical sensors have emerged as a promising alternative by leveraging the intrinsic electromagnetic interference (EMI) immunity of photons. Unlike electrons, photons are unaffected by magnetic fields, and optical fibers offer the unique capability of low-loss remote signal transmission over at least tens of meters. This allows for a paradigm where a completely passive, all-optical probe operates within the MRI isocenter, while all sensitive electronic interrogation units are safely housed outside the shielded room. Conventional optical methods, such as Fiber Bragg Gratings (FBG)\u0026nbsp;[10-13] or Photoplethysmography (PPG) [14-17], have attempted to exploit this advantage. However, these modalities predominantly operate as contact-based strain gauges or volumetric sensors, inferring vascular dynamics from indirect skin surface deformations or capillary blood volume changes [18]. From a biophysical perspective, the skin and subcutaneous tissues act as a viscoelastic low-pass filter, effectively damping the high-frequency mechanical transients generated by the arterial pulse. Consequently, while these surrogates can track heart rate, they often fail to capture the fine morphological features of deep clinical reference arteries that are essential for precise vascular compliance analysis.\u003c/p\u003e\n\u003cp\u003eTo bridge this fundamental gap, we introduce 'Ultrasound on Fiber,' a concept that synergizes the deep-tissue resolving capability of ultrasound with the electromagnetic transparency of fiber optics. Although pioneering studies have demonstrated the feasibility of encoding ultrasonic signals onto optical carriers—exemplified by applications such as ultrasound structural imaging and M-mode interventional intracardiac imaging—translating this technology into robust clinical wearables for blood pressure (BP) waveform dynamics remains challenging. Current approaches are constrained by the sensitivity required for single-vessel deformation detection and the necessity for a metal-free design; consequently, a device capable of continuous BP waveform monitoring in strong magnetic fields, such as MRI, has yet to be demonstrated [19-21]. While high-Q optical resonators offer superior sensitivity, they suffer from acute environmental instability. Conversely, robust low-Q alternatives require either prohibitive interaction lengths or multiple ultrasonic emissions to achieve a viable signal-to-noise ratio for single-vessel surface displacement. To overcome that challenge, we present a fully metal-free Fiber-Optoacoustic System (FOAS) that introduces an active detection architecture. By leveraging focused photoacoustic generation with a fiber-laser sensor, our system achieves outstanding acoustic sensitivity and stability enabled by self-heterodyne detection within a compact footprint. By directly interrogating arterial wall dynamics, the FOAS reconstructs high-fidelity BP waveforms. This capability is validated through multi-subject trials (n=10) and, crucially, safe and artifact-free operation within a 3T MRI scanner, establishing the FOAS as a transformative platform for seamless cardiovascular profiling in electromagnetically constrained environments.\u003c/p\u003e\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n"},{"header":"System Concept \u0026 Characterization ","content":"\u003cp\u003e\u003cstrong\u003e1.1 Principle of Optoacoustic Arterial Diameter Tracking\u003c/strong\u003e\u003c/p\u003e\u003cp\u003eCardiac ejection generates pulsatile pressure waves that drive cyclic arterial diameter variations, thereby encoding hemodynamic data such as BP dynamics, vascular compliance, and wave reflection [22-25] . To quantify this, our fiber-optoacoustic system (FOAS) utilizes ultrasound M-mode [26, 27] to capture wall echoes via a fiber laser sensor (Fig. 1a). Upon transmission, the focused ultrasonic waves interact with the vessel's upper and lower walls, where acoustic impedance mismatches with the surrounding tissue cause partial signal reflection. By analyzing the time-of-flight of these echoes relative to the speed of sound in tissue, we reconstruct diameter waveforms at a rate of 500 Hz with micrometer precision. Significantly, these diameter waveforms encapsulate rich physiological data, including heart rate, blood pressure, and associated hemodynamic metrics.\u003c/p\u003e\u003cp\u003eTo achieve MRI compatibility via electromagnetic immunity, we developed a compact (20 × 18 × 10 mm³), metal-free all-optical probe for wearable monitoring (Fig. 1) that seamlessly integrates optoacoustic excitation with fiber laser sensing. The system utilizes a spin-coated functional layer to convert 532 nm laser pulses, delivered via multimode fiber, into ultrasound through the photoacoustic effect [28, 29]. This concave source generates a stable 20 MHz bandwidth signal with a threefold pressure increase compared to planar counterparts, creating a 1.4 mm × 0.5 mm focal zone at a depth of 7 mm to target the radial artery (see Methods).\u003c/p\u003e\u003cp\u003eUltrasound echoes are detected at the skin surface using a 125 μm cylindrical fiber laser sensor with a self-delayed interferometric readout. Unlike conventional dual-frequency schemes, this configuration eliminates the need for dual-frequency polarized laser outputs and polarization-maintaining fibers, thereby simplifying the system architecture and enhancing stability. Furthermore, this approach recovers low- and high-frequency components typically suppressed in dual-frequency setups, effectively extending the bandwidth and improving both resolution and signal-to-noise ratio (SNR). Featuring a broad detection bandwidth of 20 MHz and a noise-equivalent pressure of 0.5 mPa/√Hz, the sensor ensures high-SNR ultrasonic detection (see Supplementary Materials). \u0026nbsp;Currently, the overall system bandwidth is primarily limited by the thickness of the photoacoustic generation coating.\u003c/p\u003e\u003cp\u003e\u003cstrong\u003e1.2 Signal Processing and Data Acquisition\u003c/strong\u003e\u003c/p\u003e\u003cp\u003eThe fiber laser ultrasound sensor, whose output is photodetected and digitized by a field-programmable gate array (FPGA)-based acquisition system (sampling rate 500 MHz, 14-bit resolution). Raw ultrasound signals undergo three processing stages: (1) The anterior and posterior wall signals are enhanced using a deconvolution algorithm. (2) The positions of the anterior and posterior walls are extracted via a peak-finding algorithm. (3) Diameter calculation from the time-of-flight difference Δt between wall echoes: d(t) = c· Δt / 2, where c = 1540 m/s is the assumed sound speed in soft tissue. Real-time display of diameter waveforms (M-mode) is achieved at 500 Hz frame rate, with beat-to-beat BP estimation performed via a compliance-based model (see Methods).\u003c/p\u003e\u003cp\u003e\u003cstrong\u003e1.3 Wearable Integration\u003c/strong\u003e\u003c/p\u003e\u003cp\u003eFor wearable deployment, the all-fiber probe is integrated into a 3D-printed housing designed for radial artery monitoring (Fig. 1a, right inset). The housing features an anatomically contoured base that conforms to the wrist curvature, with an adjustable strap enabling watch-like fixation to maintain stable probe-to-artery alignment during physiological motion. The focal length of the optoacoustic source is 7 mm, optimized for radial artery depth in adult subjects (3–5 mm beneath the skin surface). PVC Wrap at the skin interface distributes contact pressure uniformly while providing acoustic coupling via ultrasound gel. This compact, lightweight form factor (total mass \u0026lt;15 g) allows continuous monitoring across diverse scenarios without restricting limb movement or causing discomfort during extended wear. We have opted to employ the radial artery for dynamic hemodynamic measurements. Its superficial location and clear blood flow signals make it an ideal candidate for such measurements. Moreover, the radial artery exhibits a relatively stable anatomical structure, with minimal variations in its course and position across different individuals. This not only simplifies the measurement process but also enhances the standardization of the procedure, thereby ensuring the accuracy and reproducibility of the measurement outcomes.\u003c/p\u003e\u003cp\u003e\u003cstrong\u003e1.4 System Integration and MRI Compatibility\u003c/strong\u003e\u003c/p\u003e\u003cp\u003eAll optical and electronic components containing ferromagnetic materials are located outside the MRI bore, with only the wearable probe and optical fiber cables entering the 5-Gauss line (Fig. 1). Note that the probe housing is Resin, a non-magnetic, non-conductive polymer. Optical fibers (silica core, acrylate coating) exhibit negligible magnetic susceptibility (χ \u0026lt; 10⁻⁶), preventing image distortion. The complete material list is provided in Supplementary Table 1. This architecture enables artifact-free operation during active MRI scanning, as validated through simultaneous hemodynamic monitoring and gradient-echo imaging (see Results). The absence of radiofrequency (RF) coupling between the optical system and the MRI coil preserves both measurement fidelity and imaging quality, addressing a critical unmet need in MRI-guided interventions and functional imaging studies requiring continuous vital sign monitoring.\u003c/p\u003e\u003col start=\"2\"\u003e\n \u003cli\u003e\u003cstrong\u003eSystem Characterization\u003c/strong\u003e\u003c/li\u003e\n\u003c/ol\u003e\u003cp\u003eTo validate the tissue penetration and vessel tracking capabilities of the fiber-optoacoustic system, we performed \u003cem\u003eex vivo\u003c/em\u003e experiments using a radial artery phantom (Fig. 2a). A soft silicone tube (diameter: 3 mm; wall thickness: 0.7 mm; Tygon S-50-HL, Saint-Gobain) was embedded in chicken breast tissue at a depth of 1.5 mm to approximate the typical position of the radial artery in adults. The tube was positioned parallel to the tissue surface directly beneath the probe. We evaluate ultrasonic performance—including emission capability and detection sensitivity—the lumen was alternately filled with water and air. The probe operated at a 500 Hz pulse repetition rate, with acoustic echoes recorded by the fiber laser sensor at a 500 MHz sampling rate (see Methods).\u003c/p\u003e\u003cp\u003eAs shown in Fig. 2a (bottom), M-mode signals were extracted for both conditions. In the water-filled phase, distinct posterior wall echoes confirmed effective acoustic transmission, attributed to the impedance matching between water (~1.48 MRayl) and the tissue phantom (~1.5 MRayl). Conversely, in the air-filled phase, the significant impedance mismatch between the anterior wall and the air core (~0.0004 MRayl) caused nearly total reflection at the interface, resulting in the complete absence of posterior echoes.\u003c/p\u003e\u003cp\u003eQuantitative calibration (detailed in Supplementary Materials) demonstrates that the device generates a peak acoustic pressure of 1.5 MPa with a −6 dB working bandwidth of 20 MHz (see Fig. 2b, Incident US Signal). Although spectral analysis of the anterior wall echo (Fig. 2c, L1 Echo Signal) reveals a center frequency of 8 MHz and a −6 dB bandwidth of 10 MHz (reduced by high-frequency ultrasound attenuation and phase aberration in biological tissue), the acoustic signal retains sufficient spectral content for high-resolution measurement after tissue propagation. The echo pulse width indicates a spatial pulse length (SPL) of approximately 77 μm (FWHM). While SPL represents the fundamental axial resolution limit for distinguishing adjacent interfaces (e.g., differentiating anterior from posterior walls), it does not dictate the limit for dynamic displacement tracking. Under high-SNR conditions, the tracking precision is primarily constrained by the system's temporal discretization. With the implemented sampling rate of 500 MHz (2 ns time resolution) and an acoustic velocity of 1540 m/s, the system achieves an effective digital tracking spatial resolution of approximately 3 μm, enabling precise monitoring of subtle vascular wall dynamics [30].\u003c/p\u003e\u003col start=\"3\"\u003e\n \u003cli\u003e\u003cstrong\u003eIn Vivo Arterial Diameter Tracking and waveform reconstruction\u003c/strong\u003e\u003c/li\u003e\n\u003c/ol\u003e\u003cp\u003eBlood pressure is the force exerted by circulating blood against vessel walls per unit area. Its dynamic characteristics reflect the status of the cardiovascular system in real time, continuous BP monitoring is essential for the optimal diagnosis and treatment of many cardiovascular conditions. In this context, ultrasound-based measurement has demonstrated high accuracy. This method exploits the intrinsic coupling between arterial blood pressure and vessel diameter, governed by the arterial wall's elastic compliance (Fig. 3a). During systole, increased intraluminal pressure distends the artery, causing radial expansion that displaces the anterior and posterior walls in opposite directions. Conversely, during diastole, elastic recoil restores the vessel to its smaller diameter. By tracking the time-of-flight variations of ultrasound echoes reflecting from both walls, the instantaneous vessel diameter D(t), can be derived from the temporal difference between the anterior and posterior signals. This diameter is subsequently converted into blood pressure values using an exponential compliance model, calibrated against reference readings from a sphygmomanometer [30].\u003c/p\u003e\u003cp\u003eRaw M-mode ultrasound data (Fig. 3c, left) exhibit tissue echoes from skin, subcutaneous fat, and muscle layers, which obscure the arterial wall signals (signal-to-noise ratio (SNR) of 31 dB). This process converts ultrasound data to blood pressure through three key steps. (i) Signal enhancement: Frequency-domain deconvolution enhances arterial wall echoes while suppressing tissue clutter and noise. (ii) Signal extraction and Diameter calculation: Frame-by-frame peak detection extracts anterior/posterior wall positions to obtain pulsatile diameter changes (~2.1–2.3 mm range).\u0026nbsp;(iii) Blood pressure conversion:\u0026nbsp;The blood pressure measurement in this study is based on a physical model relating arterial diameter to blood pressure. The calibration procedure is carried out as follows: First, with the subject in a stable physiological state, systolic pressure (\u003cimg width=\"15\" height=\"19\" src=\"https://myfiles.space/user_files/58895_8739fc6c57c1c19a/58895_custom_files/img1769143575.png\"\u003e) and diastolic pressure (\u003cimg width=\"17\" height=\"19\" src=\"https://myfiles.space/user_files/58895_8739fc6c57c1c19a/58895_custom_files/img176914357587.png\"\u003e) are measured at the brachial artery using a standard sphygmomanometer, while the corresponding arterial cross-sectional areas at systole and diastole (\u003cimg width=\"17\" height=\"19\" src=\"https://myfiles.space/user_files/58895_8739fc6c57c1c19a/58895_custom_files/img176914357535.png\"\u003e\u0026nbsp;and \u003cimg width=\"19\" height=\"19\" src=\"https://myfiles.space/user_files/58895_8739fc6c57c1c19a/58895_custom_files/img176914357512.png\"\u003e) are simultaneously recorded. Subsequently, the vascular rigidity coefficient \u003cimg width=\"10\" height=\"19\" src=\"https://myfiles.space/user_files/58895_8739fc6c57c1c19a/58895_custom_files/img176914357511.png\"\u003e, which reflects the individual’s vascular elasticity under the current physiological condition, is calculated using the formula \u003cimg width=\"100\" height=\"30\" src=\"https://myfiles.space/user_files/58895_8739fc6c57c1c19a/58895_custom_files/img1769143576.png\"\u003e. Once calibrated, the system only needs to continuously monitor the arterial diameter waveform \u003cimg width=\"31\" height=\"19\" src=\"https://myfiles.space/user_files/58895_8739fc6c57c1c19a/58895_custom_files/img176914357650.png\"\u003e\u0026nbsp;and its minimal diastolic diameter \u003cimg width=\"19\" height=\"19\" src=\"https://myfiles.space/user_files/58895_8739fc6c57c1c19a/58895_custom_files/img176914357581.png\"\u003e\u0026nbsp;to reconstruct the continuous blood pressure waveform \u003cimg width=\"85\" height=\"19\" src=\"https://myfiles.space/user_files/58895_8739fc6c57c1c19a/58895_custom_files/img176914357520.png\"\u003e. \u003c/p\u003e\u003cp\u003eThe reconstructed pressure waveform (Fig. 3b) preserves critical morphological features essential for comprehensive cardiovascular assessment:\u003c/p\u003e\u003cp\u003e(i) Systolic Upstroke: The steep pressure rise (upstroke gradient) corresponds to left ventricular contractility and aortic valve patency. A blunted gradient may indicate impaired myocardial function or aortic stenosis [32]. (ii) Systolic Blood Pressure (SBP): The peak pressure recorded during ventricular ejection. (iii) Diastolic Blood Pressure (DBP): The nadir pressure observed during ventricular filling. (iv) Dicrotic Notch Time (DNT): Defined as the temporal interval between the systolic peak and the dicrotic notch, the DNT reflects the status of the arterial system at end-systole. High arterial compliance typically delays the notch (prolonging the DNT), whereas arterial stiffness accelerates wave reflection, causing the notch to appear earlier (shortening the DNT) [33, 34]. (v) Dicrotic Wave Index (DWI): Calculated as the ratio of the dicrotic wave amplitude to the systolic peak amplitude, this index serves as a standard for assessing arterial compliance and peripheral vascular resistance. Collectively, the preservation of the waveform contour facilitates pulse wave analysis beyond static systolic/diastolic values. These metrics are clinically significant for the early detection of hypertension, heart failure, and valvular pathologies [35-43], underscoring the potential of the FOAS device as a comprehensive wearable cardiovascular monitoring platform.\u0026nbsp;\u003c/p\u003e\u003cp\u003eWe validated the FOAS in a high-field MRI environment (GE Discovery MR750, 3.0T) to highlight the distinct advantages of the \"Ultrasound over Fiber\" concept (Fig. 3d). The system's hybrid photoacoustic-acousto-optic interface enables the excitation and detection of ultrasound strictly via optical waveguides, supporting signal transmission over distances exceeding 10 meters. This capability allows for remote monitoring from outside the MRI suite, completely decoupling sensitive interrogation electronics from the scanner's electromagnetic interference. During in-vivo testing on a healthy volunteer (24-year-old male), the device demonstrated stable performance during both \"scanner OFF\" and \"scanner ON\" sessions. The complete absence of imaging artifacts and signal degradation confirms robust bidirectional electromagnetic compatibility. Thanks to its remote interrogation unit and metal-free sensor design, the FOAS successfully captured comprehensive hemodynamic data—ranging from fundamental metrics like blood pressure and heart rate to intricate waveform morphology such as the dicrotic notch—even during active scanning, extending the horizon for safe, continuous physiological monitoring in MRI settings.\u003c/p\u003e\u003col start=\"4\"\u003e\n \u003cli\u003e\u003cstrong\u003eMultiscenario Hemodynamic Responses\u003c/strong\u003e\u003c/li\u003e\n\u003c/ol\u003e\u003cp\u003eTo demonstrate the ability of the wearable FOAS to capture rapid hemodynamic dynamics, we employed a occlusion-restitution test (see Methods). At \u003cimg width=\"45\" height=\"19\" src=\"https://myfiles.space/user_files/58895_8739fc6c57c1c19a/58895_custom_files/img176914357551.png\"\u003e\u0026nbsp;s, a sphygmomanometer cuff was inflated to 150 mmHg—exceeding systolic pressure—and subsequently released gradually at \u003cimg width=\"45\" height=\"19\" src=\"https://myfiles.space/user_files/58895_8739fc6c57c1c19a/58895_custom_files/img176914357639.png\"\u003e\u0026nbsp;s. The M-mode analysis (Fig. 4a) identifies three phases. First, during the initial inflation stage, the vascular pulse amplitude weakens, and the arterial diameter gradually expands (Fig. 4a iii). Second, when the cuff is fully inflated and pressure surpasses the systolic limit, the arterial diameter contracts and pulsation completely disappears (Fig. 4a iv). Finally, during the reperfusion transient, an abrupt diameter increase (~0.4 mm) occurs as pressure drops below systolic levels, marking the rapid reestablishment of flow and the restoration of pulsatility (Fig. 4a v). The observed vascular recovery speed is indicative of vessel elasticity and microcirculatory condition. \u003c/p\u003e\u003cp\u003eBenefiting from high resolution and signal-to-noise ratio, this device goes beyond basic heart rate and blood pressure estimation to provide detailed pulse waveforms through direct vessel measurement. This enables precise cardiovascular health assessment during daily routine and monitoring of medication impacts. We present two typical tests here: In the exercise scenario (Fig. 4b), continuous monitoring immediately after 10 minutes of moderate-intensity cycling showed increases of 27 mmHg (SBP) and 18 mmHg (DBP), with average heart rate rising from 88 to 106 bpm. The waveform morphology shifted systematically: peripheral vasodilation reduced pulse wave reflection, leading to attenuated tidal and dicrotic components and a sharper single-peak morphology; the systolic-to-dicrotic interval DNT prolonged (0.2 s to 0.22 s, 10% increase), indicating reduced peripheral resistance and enhanced arterial compliance [44, 45]. These observations align with exercise-induced hemodynamic models and demonstrate the device’s sensitivity to rapid dynamics and waveform morphology.\u003c/p\u003e\u003cp\u003eIn the caffeine intake challenge (Fig. 4c), re-measurement 30 minutes after consuming 200 mL coffee revealed modest yet consistent BP elevations (SBP +6 mmHg; DBP +4 mmHg) with increased heart rate. In contrast to exercise, the waveform exhibited enhanced dicrotic components and a deepened notch, with shortened DNT (0.24 s to 0.16 s, 33% decrease), reflecting caffeine-induced vasoconstriction and increased pulse wave reflection due to higher peripheral resistance [46, 47]. Simultaneously, caffeine stimulation enhances myocardial contractility, thereby increasing stroke volume and ejection velocity [48].\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eThe opposite trends of dicrotic amplitude and DNT across the pre/post caffeine consumption and pre/post exercise indicate that the device discriminates vasodilatory versus vasoconstrictive states, enabling physiologically grounded interpretation at beat-to-beat resolution.\u003c/p\u003e\u003cp\u003e\u003cstrong\u003e5. Individual Variability in BP Waveform Morphology and Vascular Compliance Assessment\u003c/strong\u003e\u003c/p\u003e\u003cp\u003eTo evaluate system repeatability and inter-subject discriminability, we performed continuous noninvasive monitoring on a cohort of 10 healthy volunteers (demographics in Supplementary Table 2). While all subjects maintained normotensive blood pressure, the extracted beat-to-beat waveforms revealed distinct inter-individual morphological heterogeneity, with the system consistently resolving fine features such as the dicrotic notch across all age groups (Fig. 5a).\u003c/p\u003e\u003cp\u003eTo decode these morphological variations, we analyzed the Dicrotic Notch Time (DNT)—a metric governed by left-ventricular ejection time, peripheral vascular resistance, and the timing of wave reflections [33, 34]. Comparison of DNT against key hemodynamic parameters revealed two significant population-level trends (Fig. 5b). First, DNT is negatively correlated with Diastolic Blood Pressure (DBP) (Fig. 5b-(1)). \u0026nbsp;Physiologically, reduced peripheral resistance and higher vascular compliance delay the return of reflected waves (prolonging DNT) while facilitating diastolic runoff, thereby lowering DBP. Conversely, elevated resistance or stiffness accelerates pulse wave velocity and reflection strength, resulting in an earlier notch appearance (shortened DNT) and higher pressure. Second, DNT exhibits a strong negative correlation with Heart Rate (HR) (Fig. 5b-(2)). This aligns with the cardiac cycle dynamics, where higher heart rates shorten the ejection phase and accelerate aortic valve closure, effectively reducing the systolic-peak-to-notch interval.\u003c/p\u003e\u003cp\u003eAlthough the Dicrotic Wave Index (DWI) displayed pronounced individual-specific variance (Fig. 5b-(3)), a discernible inverse relationship with age was observed. Specifically, DWI tended to decrease in older subjects, a phenomenon likely attributable to age-related arterial stiffening, which diminishes the vessel's elastic recoil capacity and consequently attenuates the dicrotic wave. Collectively, these morphological insights—alongside the robust correlations observed between DNT and hemodynamic parameters (DBP, HR)—confirm that the extracted waveform features serve as valuable biomarkers. These findings validate the FOAS's capability to distinguish subtle inter-subject differences in vascular compliance and peripheral resistance, consistent with arterial wave propagation theory.\u003c/p\u003e"},{"header":"Discussion and Conclusion","content":"\u003cp\u003eIn this work, we achieved a fully integrated, electromagnetically neutral system by realizing a hybrid fiber-optoacoustic Interface, which synergizes wavefront-engineered photoacoustic generation with active fiber-laser detection. This work established \"Ultrasound over Fiber\" as a potential methodology for continuous hemodynamic monitoring, specifically engineered to bridge the gap between deep-tissue sensing and electromagnetic compatibility. This metal-free architecture not only ensures intrinsic immunity to electromagnetic interference (EMI), redefining physiological tracking in high-field MRI environments, but also fundamentally surpasses the limitations of conventional optical sensors (e.g., PPG) [\u003cspan additionalcitationids=\"CR15 CR16\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Unlike surface-limited photometric techniques, our system leverages ultrasonic M-mode to directly interrogate subcutaneous arterial mechanics [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. By bypassing the viscoelastic filtering of skin and tissue, it captures precise vascular dimensional dynamics and reconstructs high-fidelity blood pressure waveforms that faithfully reflect central cardiovascular status.\u003c/p\u003e \u003cp\u003eTo achieve such high performance within a compact wearable footprint, we introduced a triad of innovations designed to circumvent the limitations of conventional all-optical ultrasound sensors: (1) A concave photoabsorptive interface allows for acoustic energy focusing, thereby maximizing opto-mechanical coupling efficiency; (2) Leveraging the high sensitivity of a fiber laser sensor, we attained a 31 dB in vivo SNR without compromising environmental stability; (3) A deconvolution-based processing pipeline suppresses clutter from non-vascular tissues to precisely isolate arterial wall signals. The system's robustness was confirmed through artifact-free operation within an active 3T MRI scanner, representing a paradigm shift in unobstructed vital sign monitoring for patients in high-magnetic-field settings. Looking forward, the translation of this all-optical modality into clinical workflows holds the promise of addressing the blind spot in current critical care [\u003cspan additionalcitationids=\"CR20\" citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Currently, MRI-guided interventions (e.g., cardiac catheterization, tumor ablation) often lack real-time hemodynamic feedback because standard electrical sensors introduce heating hazards or image artifacts. Our \"electromagnetically transparent\" technology may fill this void, potentially enabling a new method of intraoperative monitoring where patient stability is tracked continuously without interrupting imaging sequences. Furthermore, consistent with emerging trends in wearable electronics, the high-fidelity acoustic signatures captured by our device are ripe for AI-driven analytics. By coupling these rich optical-ultrasound datasets with deep learning algorithms, future systems could automate the extraction of latent digital biomarkers, such as stroke volume variability and vascular aging indices, effectively transforming the device from a passive monitor into an intelligent diagnostic companion for precision medicine.\u003c/p\u003e \u003cp\u003eDespite these encouraging prospects, this study identifies directions for future evolution. First, absolute BP quantification still relies on initial cuff-based calibration. Future work will explore calibration-free models based on pulse wave velocity (PWV) or arterial wall elasticity to enhance system autonomy. Second, while the current single-point measurement is effective, extending this technology to sensor arrays could enable tomographic ultrasound imaging for enhanced cardiovascular profiling. Third, to extend monitoring from major arteries to microvasculature, future iterations will focus on broadening the spectral response by optimizing acoustic impedance matching and miniaturizing active elements to resolve higher-frequency components.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003e\u003c/p\u003e\n\u003ch2\u003e\u003cstrong\u003e1. Coating Structure and Materials\u003c/strong\u003e\u003c/h2\u003e\n\u003cp\u003e\u003c/p\u003e\n\u003cp\u003eThe optoacoustic source employs a hybrid composite coating. The coating comprises two functional material [\u003cspan class=\"CitationRef\"\u003e49\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e60\u003c/span\u003e] (Supplementary Fig.\u0026nbsp;1a):\u003c/p\u003e\n\u003cp\u003e\u003c/p\u003e\n\u003col style=\"list-style-type: lower-roman;\"\u003e\n \u003cli\u003eAbsorption material: Carbon black provides high optical absorption and rapid photothermal conversion, effectively matching the laser pulse duration to maximize thermoelastic efficiency.\u003c/li\u003e\n \u003cli\u003eExpansion material: Polydimethylsiloxane (PDMS, Sylgard 184, Dow Corning, 10:1 base-to-curing agent ratio) provides acoustic impedance matching to tissue (~\u0026thinsp;1.5 MRayl vs. 1.4 MRayl for soft tissue. Upon nanosecond laser pulse irradiation (532 nm, 8 ns FWHM, 200 \u0026micro;J/pulse, 500 Hz repetition rate), localized heating in the absorption layer induces rapid thermal expansion of the PDMS, launching acoustic waves into the surrounding medium.\u003c/li\u003e\n\u003c/ol\u003e\n\u003cp\u003e\u003c/p\u003e\n\u003cp\u003eThe mass-to-volume ratio of the two functional materials is formulated at 25 mg carbon powder per 0.1 mL PDMS. The blended composite is deposited as a uniform thin-film coating through a two-stage spin-coating protocol: an initial spreading phase at 900 rpm for 15 seconds ensures homogeneous distribution, followed by a thickness-control phase at 5000 rpm for 60 seconds. The coating was cured in a 100\u0026deg;C vacuum drying oven for 2 hours. This process yields a consolidated coating with thickness ranging from 90 to 100 \u0026micro;m.\u003c/p\u003e\n\u003ch3\u003e2. Acoustic Focusing and Beam Shaping\u003c/h3\u003e\n\u003cp\u003eTo concentrate acoustic energy at the radial artery depth and reduce clutter from surrounding tissues, a plano-concave acoustic mold is integrated into the probe housing. The mold is fabricated from transparent resin using 3D printing technology. The prepared optoacoustic coating is applied to the concave surface of the mold using UV-curable adhesive and subsequently solidified. After curing, the acoustic source is oriented with its concave surface facing the biological tissue.\u003c/p\u003e\n\u003cp\u003eThe 7 mm radius of curvature is designed based on the probe standoff distance (4 mm from the mold to the skin surface) and the target artery depth (3\u0026ndash;5 mm beneath the skin), placing the acoustic focus at ~\u0026thinsp;7 mm from the mold surface. Acoustic field mapping via calibrated needle hydrophone (0.2 mm aperture, Precision Acoustics) on a motorized XY stage (0.02 mm step size, 6 mm \u0026times; 6 mm scan area) confirms a \u0026minus;\u0026thinsp;6 dB focal zone of 0.5 mm (lateral) \u0026times; 1.4 mm (axial) at 7 mm depth, corresponding to a numerical aperture of 0.21 (Supplementary Fig.\u0026nbsp;1e).\u003c/p\u003e\n\u003cp\u003eFrequency analysis of the acoustic waveform reveals a center frequency of 8 MHz with a \u0026minus;\u0026thinsp;6 dB bandwidth of 8\u0026ndash;25 MHz, sufficient for arterial wall imaging with ~\u0026thinsp;40 \u0026micro;m axial resolution (calculated as 0.5 \u0026times; speed of sound / bandwidth\u0026thinsp;=\u0026thinsp;0.5 \u0026times; 1540 m/s / 20 MHz). Peak pressure at the focus reaches 1.5 MPa (measured at 200 \u0026micro;J laser energy), representing a threefold enhancement compared to the unfocused configuration (0.5 MPa, Supplementary Fig.\u0026nbsp;1c, e). The focused beam also exhibits\u0026thinsp;\u0026gt;\u0026thinsp;10 dB signal-to-noise ratio improvement at the center frequency, reducing interference from subcutaneous fat and muscle layers.\u003c/p\u003e\n\u003ch3\u003e3. Reproducibility and Stability of the Optoacoustic Source\u003c/h3\u003e\n\u003cp\u003eTo assess batch-to-batch consistency, five optoacoustic sources were fabricated using the optimized spin-coating protocol. Acoustic pressure measurements at 200 \u0026micro;J laser energy yielded 1.48\u0026thinsp;\u0026plusmn;\u0026thinsp;0.12 MPa (coefficient of variation 8%), confirming reproducible performance. Long-term stability was evaluated by operating a single source at maximum safe energy (200 \u0026micro;J, corresponding to ~\u0026thinsp;6 mJ/cm\u0026sup2; fluence on the coating) for 30 minutes (9 \u0026times; 10\u003csup\u003e5\u003c/sup\u003e pulses). The acoustic amplitude remained within 4000\u0026thinsp;\u0026plusmn;\u0026thinsp;500 a.u. (12% variation), with no visible coating degradation or delamination (Supplementary Fig.\u0026nbsp;1d).\u003c/p\u003e\n\u003ch3\u003e4. Safety Compliance\u003c/h3\u003e\n\u003cp\u003eThe acoustic output complies with FDA guidelines for diagnostic ultrasound (FDA 510(k) Track 3). The mechanical index MI\u0026thinsp;=\u0026thinsp;peak negative pressure / \u0026radic;center frequency\u0026thinsp;=\u0026thinsp;1.5 MPa / \u0026radic;15 MHz\u0026thinsp;=\u0026thinsp;0.39\u0026thinsp;\u0026lt;\u0026thinsp;1.9 (regulatory limit). The spatial-peak pulse-average intensity I_SPPA = (peak pressure)\u0026sup2; / (2 \u0026times; acoustic impedance \u0026times; pulse repetition frequency) = (1.5 MPa)\u0026sup2; / (2 \u0026times; 1.5 MRayl \u0026times; 1 kHz)\u0026thinsp;=\u0026thinsp;120 W/cm\u0026sup2; \u0026lt; 190 W/cm\u0026sup2; (regulatory limit). Thermal effects are negligible due to the low duty cycle (8 ns pulse width \u0026times; 500 Hz\u0026thinsp;=\u0026thinsp;4 \u0026times; 10⁻⁶ duty cycle), resulting in \u0026lt;\u0026thinsp;0.1\u0026deg;C temperature rise at the skin surface (estimated via finite element thermal modeling).\u003c/p\u003e\n\u003cp\u003eThe human blood pressure monitoring experiments were conducted in accordance with a protocol approved by the Ethics Committee of Jinan University (Approval Number: JNUKY-2023-0151, n\u0026thinsp;=\u0026thinsp;10 healthy volunteers). Total ten healthy adult male and female volunteers, aged from 22 to 37 years, participated in the study. Sex and gender were not taken into consideration in this experimental design and all participants were randomly selected.\u003c/p\u003e\n\u003ch3\u003e5. Fiber Laser Ultrasound Sensor\u003c/h3\u003e\n\u003cp\u003eAcoustic echo detection employs a distributed Bragg reflector (DBR) fiber laser sensor embedded in an erbium-ytterbium codoped fiber core. The sensor architecture and operating principle are illustrated in Supplementary Fig.\u0026nbsp;2a. The laser cavity, contains two wavelength matched Bragg gratings (central wavelength: 1550 nm, \u0026gt;\u0026thinsp;25 dB in reflectivity), inscribed in erbium-ytterbium doped fiber (EYDF, absorption coefficient over 1000 dB/m at 1530 nm) using phase-mask UV lithography. The laser cavity length is about 3 mm, ensuring a single-longitudinal-mode operation. The cavity is pumped by a 980 nm laser diode via a wavelength-division multiplexer (WDM), generating single-frequency lasing at 1550 nm with a kHz-level linewidth.\u003c/p\u003e\n\u003cp\u003eIncident acoustic waves induce strain in the fiber through photoelastic effect, modulating the effective refractive index and physical length of the optical fiber. The induced laser frequency variation was then readout via a delayed self-heterodyne method, which the sensor achieves an average noise-equivalent pressure density (NEP) of ~\u0026thinsp;15 Pa over a bandwidth of 1\u0026ndash;40 MHz. Note that laser oscillation effectively enhances acoustic responsivity by three orders of magnitude. The cylindrical fiber geometry (125 \u0026micro;m diameter) provides omnidirectional reception with a 180\u0026deg; acceptance angle in the plane perpendicular to the fiber axis, relaxing alignment constraints for wearable deployment. The fiber laser sensor enables seamless integration into a compact probe housing alongside the optoacoustic source.\u003c/p\u003e\n\u003cp\u003eAcoustic sensitivity is calibrated using a broadband piezoelectric transducer (V214-BC-RM, Olympus-NDT, center frequency 50 MHz) in a water tank. The transducer generates plane waves of known pressure amplitude (measured via calibrated hydrophone), and the fiber sensor output is recorded interferometrically based on the IQ demodulation module described in the Supplementary Materials.\u003c/p\u003e\n\u003ch3\u003e6. Phantom Fabrication and Acoustic Properties\u003c/h3\u003e\n\u003cp\u003eThe chicken breast tissue phantom was prepared from fresh chicken breast (purchased from local market, stored at 4\u0026deg;C, used within 24 hours). The tissue was cut into 50 mm \u0026times; 20 mm \u0026times; 20 mm blocks, and a channel (3 mm diameter) was created at 2 mm depth using a biopsy punch. The silicone tube was inserted into the channel and secured with tissue adhesive. Acoustic properties of the chicken breast tissue were characterized via through-transmission measurements using a broadband transducer (V214-BB-RM, Olympus): speed of sound c\u0026thinsp;=\u0026thinsp;1560\u0026thinsp;\u0026plusmn;\u0026thinsp;20 m/s, attenuation coefficient \u0026alpha;(f)\u0026thinsp;=\u0026thinsp;1.15 * f dB/cm (with f in MHz), consistent with literature values for muscle tissue.\u003c/p\u003e\n\u003ch3\u003e7. Blood Pressure Waveforms During Pressurization\u003c/h3\u003e\n\u003cp\u003eWe performed continuous radial arterial monitoring on a healthy volunteer (male, 24 years, no history of cardiovascular disease) over 60 seconds. The device was positioned over the right radial artery with and secured via an elastic wristband, while a reference sphygmomanometer (Omron U724J) was placed on the right upper arm for pressurization (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea). The blood pressure cuff was inflated to 150 mmHg at t\u0026thinsp;=\u0026thinsp;15 s and gradually deflated from t\u0026thinsp;=\u0026thinsp;35 s. M-mode imaging revealed three characteristic phases: transient arterial expansion and reduced pulse amplitude during early inflation, suppressed pulsations under supra-systolic pressure, and a 0.4 mm step-change in diameter with restored flow upon deflation.\u003c/p\u003e\n\u003ch3\u003e8. Signal Processing\u003c/h3\u003e\n\u003cp\u003eRaw M-mode ultrasound data (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ec, left) exhibit tissue echoes from skin, subcutaneous fat, and muscle layers, which obscure the arterial wall signals. To enhance the signal-to-noise ratio and suppress non-arterial clutter, we applied frequency-domain deconvolution using the anterior wall echo as the point spread function (PSF). The PSF is windowed via a Hamming function to prevent spectral leakage, and the entire M-mode image is deconvolved in the Fourier domain. Spectral components with high correlation to the arterial wall\u0026apos;s frequency signature are enhanced, while low-frequency tissue clutter (\u0026lt;\u0026thinsp;5 MHz) and high-frequency noise (\u0026gt;\u0026thinsp;20 MHz) are suppressed. The deconvolved image (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ec, bottom left) exhibits sharper, more concentrated arterial wall echoes, facilitating robust peak detection. Through peak detection extracts the depth positions of anterior and posterior walls frame-by-frame (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ec, middle panels), and their difference yields the diameter time series (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ec, right panel, 2.1\u0026ndash;2.3 mm range with ~\u0026thinsp;0.2 mm pulsatile amplitude). Using the calibrated compliance model:\u003c/p\u003e\n\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\n \u003cdiv class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e$$\\:P\\left(t\\right)={P}_{d}\\cdot\\:{e}^{\\alpha\\:\\left(\\frac{D\\left(t\\right)}{{D}_{d}}-1\\right)}\\:\\:\\:$$\u003c/div\u003e\n \u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003ewith parameters \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{P}_{d}=68\\:mmHg\\:\\)\u003c/span\u003e\u003c/span\u003e(diastolic pressure from sphygmomanometer), \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{D}_{d}=2.12mm\\)\u003c/span\u003e\u003c/span\u003e (diastolic diameter), and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\alpha\\:\\)\u003c/span\u003e\u003c/span\u003e (arterial stiffness coefficient), the diameter waveform is converted to the blood pressure waveform shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ec, bottom right. Since the vascular rigidity coefficient \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\alpha\\:\\)\u003c/span\u003e\u003c/span\u003e and diastolic pressure \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{P}_{d}\\)\u003c/span\u003e\u003c/span\u003e do not differ significantly across different arterial sites within the same subject, this one-time calibration parameter is applicable for subsequent blood pressure waveform reconstruction at other anatomical locations (such as the wrist, neck, and foot) without the need for frequent recalibration, unless the subject undergoes significant physiological or vascular structural changes.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cspan\u003eThis study has been approved by the relevant institutional ethics committee (IRB of Jinan University, Approval No. JNUKY-2023-0151). All participants were informed of the study content and provided written informed consent prior to participation.\u003c/span\u003e\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eAcknowledgements\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study was supported by funding from Natural Science Foundation of China (no. 62322506 to Y.L., no. 62275104 to Y.L., no. 62135006 to B.-O. G.), National Key Technologies R\u0026amp;D Program of China (no. 2023YFF0715302 to L.J.), Research Teams Project of Guangdong Pearl River Talents Program (no. 2019BT02X105 to B.-O. G.), Science and Technology Projects in Guangzhou (no. 2024B03J0254 to L.J.).\u0026nbsp;Noncommunicable Chronic Diseases-National Science and Technology Maior Project (2023ZD0505500).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of Interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no conflict of interest.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eA. Bergholz, G. Greiwe, K. Kouz, and B. Saugel, \u0026quot;Continuous Blood Pressure Monitoring in Patients Having Surgery: A Narrative Review,\u0026quot; (in eng), \u003cem\u003eMedicina (Kaunas), \u003c/em\u003evol. 59, no. 7, Jul 14 2023, doi: 10.3390/medicina59071299.\u003c/li\u003e\n\u003cli\u003eB. H. McGhee and E. J. Bridges, \u0026quot;Monitoring arterial blood pressure: what you may not know,\u0026quot; (in eng), \u003cem\u003eCrit Care Nurse, \u003c/em\u003evol. 22, no. 2, pp. 60-4, 66-70, 73 passim, Apr 2002.\u003c/li\u003e\n\u003cli\u003eG. J. Langewouters, J. J. Settels, R. Roelandt, and K. H. Wesseling, \u0026quot;Why use Finapres or Portapres rather than intra-arterial or intermittent non-invasive techniques of blood pressure measurement?,\u0026quot; (in eng), \u003cem\u003eJ Med Eng Technol, \u003c/em\u003evol. 22, no. 1, pp. 37-43, Jan-Feb 1998, doi: 10.3109/03091909809009997.\u003c/li\u003e\n\u003cli\u003eA. S. Meidert and B. Saugel, \u0026quot;Techniques for Non-Invasive Monitoring of Arterial Blood Pressure,\u0026quot; (in eng), \u003cem\u003eFront Med (Lausanne), \u003c/em\u003evol. 4, p. 231, 2017, doi: 10.3389/fmed.2017.00231.\u003c/li\u003e\n\u003cli\u003eL. Geddes, M. Voelz, C. Combs, D. Reiner, and C. F. Babbs, \u0026quot;Characterization of the oscillometric method for measuring indirect blood pressure,\u0026quot; \u003cem\u003eAnnals of biomedical engineering, \u003c/em\u003evol. 10, no. 6, pp. 271-280, 1982.\u003c/li\u003e\n\u003cli\u003eM. Lin\u003cem\u003e et al.\u003c/em\u003e, \u0026quot;A fully integrated wearable ultrasound system to monitor deep tissues in moving subjects,\u0026quot; \u003cem\u003eNature biotechnology, \u003c/em\u003evol. 42, no. 3, pp. 448-457, 2024.\u003c/li\u003e\n\u003cli\u003eJ.-\u0026Eacute;. S. Kenny, \u0026quot;Functional hemodynamic monitoring with a wireless ultrasound patch,\u0026quot; \u003cem\u003eJournal of Cardiothoracic and Vascular Anesthesia, \u003c/em\u003evol. 35, no. 5, pp. 1509-1515, 2021.\u003c/li\u003e\n\u003cli\u003eJ. R. Sempionatto\u003cem\u003e et al.\u003c/em\u003e, \u0026quot;An epidermal patch for the simultaneous monitoring of haemodynamic and metabolic biomarkers,\u0026quot; \u003cem\u003eNature Biomedical Engineering, \u003c/em\u003evol. 5, no. 7, pp. 737-748, 2021.\u003c/li\u003e\n\u003cli\u003eJ. Li\u003cem\u003e et al.\u003c/em\u003e, \u0026quot;Recent Advances in Materials, Devices and Algorithms Toward Wearable Continuous Blood Pressure Monitoring,\u0026quot; \u003cem\u003eACS Nano, \u003c/em\u003evol. 18, no. 27, pp. 17407-17438, 2024, doi: 10.1021/acsnano.4c04291.\u003c/li\u003e\n\u003cli\u003eJ. Guo\u003cem\u003e et al.\u003c/em\u003e, \u0026quot;Fiber Cladding Waveguide Phase-Shifted FBG Encapsulated in Flexible Material for Estimating Human Blood Pressure,\u0026quot; \u003cem\u003eJournal of Lightwave Technology, \u003c/em\u003evol. 43, no. 7, pp. 3261-3268, 2025, doi: 10.1109/jlt.2024.3515043.\u003c/li\u003e\n\u003cli\u003eD. Jia\u003cem\u003e et al.\u003c/em\u003e, \u0026quot;A Fiber Bragg Grating Sensor for Radial Artery Pulse Waveform Measurement,\u0026quot; \u003cem\u003eIEEE Transactions on Biomedical Engineering, \u003c/em\u003evol. 65, no. 4, pp. 839-846, 2018, doi: 10.1109/tbme.2017.2722008.\u003c/li\u003e\n\u003cli\u003eY. Haseda, J. Bonefacino, H.-Y. Tam, S. Chino, S. Koyama, and H. Ishizawa, \u0026quot;Measurement of pulse wave signals and blood pressure by a plastic optical fiber FBG sensor,\u0026quot; \u003cem\u003eSensors, \u003c/em\u003evol. 19, no. 23, p. 5088, 2019.\u003c/li\u003e\n\u003cli\u003eZ. Tang, S. Wang, and C. Shi, \u0026quot;Development of a hybrid force-displacement sensor based on fiber Bragg grating for radial artery pulse waveform measurement,\u0026quot; \u003cem\u003eIEEE sensors journal, \u003c/em\u003evol. 21, no. 18, pp. 20045-20054, 2021.\u003c/li\u003e\n\u003cli\u003eS. Haddad, A. Boukhayma, and A. Caizzone, \u0026quot;Continuous PPG-Based Blood Pressure Monitoring Using Multi-Linear Regression,\u0026quot; (in eng), \u003cem\u003eIEEE J Biomed Health Inform, \u003c/em\u003evol. 26, no. 5, pp. 2096-2105, May 2022, doi: 10.1109/jbhi.2021.3128229.\u003c/li\u003e\n\u003cli\u003eG. Chen, L. Zou, and Z. Ji, \u0026quot;A review: Blood pressure monitoring based on PPG and circadian rhythm,\u0026quot; (in eng), \u003cem\u003eAPL Bioeng, \u003c/em\u003evol. 8, no. 3, p. 031501, Sep 2024, doi: 10.1063/5.0206980.\u003c/li\u003e\n\u003cli\u003eP. H. Charlton\u003cem\u003e et al.\u003c/em\u003e, \u0026quot;Assessing hemodynamics from the photoplethysmogram to gain insights into vascular age: a review from VascAgeNet,\u0026quot; \u003cem\u003eAmerican Journal of Physiology-Heart and Circulatory Physiology, \u003c/em\u003evol. 322, no. 4, pp. H493-H522, 2022.\u003c/li\u003e\n\u003cli\u003eG. Chan\u003cem\u003e et al.\u003c/em\u003e, \u0026quot;Multi-site photoplethysmography technology for blood pressure assessment: challenges and recommendations,\u0026quot; \u003cem\u003eJournal of clinical medicine, \u003c/em\u003evol. 8, no. 11, p. 1827, 2019.\u003c/li\u003e\n\u003cli\u003eR. Dagenais and G. D. Mitsis, \u0026quot;Non-invasive estimation of arterial blood pressure fluctuations using a peripheral photoplethysmograph inside the MRI scanner,\u0026quot; (in eng), \u003cem\u003eAnnu Int Conf IEEE Eng Med Biol Soc, \u003c/em\u003evol. 2023, pp. 1-4, Jul 2023, doi: 10.1109/embc40787.2023.10340020.\u003c/li\u003e\n\u003cli\u003eR. J. Colchester\u003cem\u003e et al.\u003c/em\u003e, \u0026quot;All-Optical Rotational Ultrasound Imaging,\u0026quot; (in eng), \u003cem\u003eSci Rep, \u003c/em\u003evol. 9, no. 1, p. 5576, Apr 3 2019, doi: 10.1038/s41598-019-41970-z.\u003c/li\u003e\n\u003cli\u003eK. Pham\u003cem\u003e et al.\u003c/em\u003e, \u0026quot;Broadband All-Optical Plane-Wave Ultrasound Imaging System Based on a Fabry-Perot Scanner,\u0026quot; (in eng), \u003cem\u003eIEEE Trans Ultrason Ferroelectr Freq Control, \u003c/em\u003evol. 68, no. 4, pp. 1007-1016, Apr 2021, doi: 10.1109/tuffc.2020.3028749.\u003c/li\u003e\n\u003cli\u003eE. Z. Zhang and P. C. Beard, \u0026quot;A miniature all-optical photoacoustic imaging probe,\u0026quot; in \u003cem\u003ePhotons plus ultrasound: imaging and sensing 2011\u003c/em\u003e, 2011, vol. 7899: SPIE, pp. 291-296. \u003c/li\u003e\n\u003cli\u003eJ. E. Sharman\u003cem\u003e et al.\u003c/em\u003e, \u0026quot;Validation of non-invasive central blood pressure devices: ARTERY Society task force consensus statement on protocol standardization,\u0026quot; \u003cem\u003eEuropean Heart Journal, \u003c/em\u003evol. 38, no. 37, pp. 2805-2812, 2017, doi: 10.1093/eurheartj/ehw632.\u003c/li\u003e\n\u003cli\u003eP. Salvi\u003cem\u003e et al.\u003c/em\u003e, \u0026quot;Non-invasive assessment of arterial stiffness: pulse wave velocity, pulse wave analysis and carotid cross-sectional distensibility: comparison between methods,\u0026quot; \u003cem\u003eJournal of Clinical Medicine, \u003c/em\u003evol. 11, no. 8, p. 2225, 2022.\u003c/li\u003e\n\u003cli\u003eG. Kober and J. O. Arndt, \u0026quot;[Pressure-diameter relationship in the common carotid artery of conscious man],\u0026quot; (in ger), \u003cem\u003ePflugers Arch, \u003c/em\u003evol. 314, no. 1, pp. 27-39, 1970, doi: 10.1007/bf00587044. Die Druck-Durchmesser-Beziehung der A. carotis communis des wachen Menschen.\u003c/li\u003e\n\u003cli\u003eJ. O. Arndt and G. Kober, \u0026quot;[Pressure-diameter relationship of the intact femoral artery in conscious man, and its influencing by noradrenaline infusions],\u0026quot; (in ger), \u003cem\u003ePflugers Arch, \u003c/em\u003evol. 318, no. 2, pp. 130-46, 1970, doi: 10.1007/bf00586492. Die Druck-Durchmesser-Beziehung der intakten A. femoralis des wachen Menschen und ihre Beeinflussung durch Noradrenalin-Infusionen.\u003c/li\u003e\n\u003cli\u003eX. Gao\u003cem\u003e et al.\u003c/em\u003e, \u0026quot;A wearable echomyography system based on a single transducer,\u0026quot; \u003cem\u003eNature Electronics, \u003c/em\u003evol. 7, no. 11, pp. 1035-1046, 2024, doi: 10.1038/s41928-024-01271-4.\u003c/li\u003e\n\u003cli\u003eM. Lin\u003cem\u003e et al.\u003c/em\u003e, \u0026quot;A fully integrated wearable ultrasound system to monitor deep tissues in moving subjects,\u0026quot; \u003cem\u003eNature Biotechnology, \u003c/em\u003evol. 42, no. 3, pp. 448-457, 2023, doi: 10.1038/s41587-023-01800-0.\u003c/li\u003e\n\u003cli\u003eE. Svanstr\u0026ouml;m, T. Linder, and T. L\u0026ouml;fqvist, \u0026quot;Analytical one-dimensional model for laser-induced ultrasound in planar optically absorbing layer,\u0026quot; (in eng), \u003cem\u003eUltrasonics, \u003c/em\u003evol. 54, no. 3, pp. 888-93, Mar 2014, doi: 10.1016/j.ultras.2013.10.019.\u003c/li\u003e\n\u003cli\u003eR. K. Poduval\u003cem\u003e et al.\u003c/em\u003e, \u0026quot;Optical fiber ultrasound transmitter with electrospun carbon nanotube-polymer composite,\u0026quot; (in eng), \u003cem\u003eAppl Phys Lett, \u003c/em\u003evol. 110, no. 22, p. 223701, May 29 2017, doi: 10.1063/1.4984838.\u003c/li\u003e\n\u003cli\u003eC. Wang\u003cem\u003e et al.\u003c/em\u003e, \u0026quot;Monitoring of the central blood pressure waveform via a conformal ultrasonic device,\u0026quot; \u003cem\u003eNature Biomedical Engineering, \u003c/em\u003evol. 2, no. 9, pp. 687-695, 2018, doi: 10.1038/s41551-018-0287-x.\u003c/li\u003e\n\u003cli\u003eS. Zhou\u003cem\u003e et al.\u003c/em\u003e, \u0026quot;Clinical validation of a wearable ultrasound sensor of blood pressure,\u0026quot; \u003cem\u003eNature biomedical engineering, \u003c/em\u003evol. 9, no. 6, pp. 865-881, 2025.\u003c/li\u003e\n\u003cli\u003eJ. Manolas, P. Wirz, and W. Rutishauser, \u0026quot;Relationship between duration of systolic upstroke of apexcardiogram and internal indexes of myocardial function in man,\u0026quot; \u003cem\u003eAmerican Heart Journal, \u003c/em\u003evol. 91, no. 6, pp. 726-734, 1976.\u003c/li\u003e\n\u003cli\u003eA. S. Alhakak, J. R. Teerlink, J. Lindenfeld, M. B\u0026ouml;hm, G. M. Rosano, and T. Biering-S\u0026oslash;rensen, \u0026quot;The significance of left ventricular ejection time in heart failure with reduced ejection fraction,\u0026quot; \u003cem\u003eEuropean journal of heart failure, \u003c/em\u003evol. 23, no. 4, pp. 541-551, 2021.\u003c/li\u003e\n\u003cli\u003eT. Biering‐S\u0026oslash;rensen\u003cem\u003e et al.\u003c/em\u003e, \u0026quot;Left ventricular ejection time is an independent predictor of incident heart failure in a community‐based cohort,\u0026quot; \u003cem\u003eEuropean journal of heart failure, \u003c/em\u003evol. 20, no. 7, pp. 1106-1114, 2018.\u003c/li\u003e\n\u003cli\u003eZ. Song\u003cem\u003e et al.\u003c/em\u003e, \u0026quot;Bioinspired microstructured pressure sensor based on a janus graphene film for monitoring vital signs and cardiovascular assessment,\u0026quot; \u003cem\u003eAdvanced Electronic Materials, \u003c/em\u003evol. 4, no. 11, p. 1800252, 2018.\u003c/li\u003e\n\u003cli\u003eY. Liu, F. Meng, Y. Zhou, S. M. Mugo, and Q. Zhang, \u0026quot;Graphene oxide films prepared using gelatin nanofibers as wearable sensors for monitoring cardiovascular health,\u0026quot; \u003cem\u003eAdvanced Materials Technologies, \u003c/em\u003evol. 4, no. 11, p. 1900540, 2019.\u003c/li\u003e\n\u003cli\u003eJ. Wang\u003cem\u003e et al.\u003c/em\u003e, \u0026quot;Diaphragm‐based optical fiber sensor for pulse wave monitoring and cardiovascular diseases diagnosis,\u0026quot; \u003cem\u003eJournal of Biophotonics, \u003c/em\u003evol. 12, no. 10, 2019, doi: 10.1002/jbio.201900084.\u003c/li\u003e\n\u003cli\u003eL. Li\u003cem\u003e et al.\u003c/em\u003e, \u0026quot;Automatic and continuous blood pressure monitoring via an optical-fiber-sensor-assisted smartwatch,\u0026quot; \u003cem\u003ePhotoniX, \u003c/em\u003evol. 4, no. 1, 2023, doi: 10.1186/s43074-023-00099-z.\u003c/li\u003e\n\u003cli\u003eU. Pingali, C. Nutalapati, and S. Gundagani, \u0026quot;Effect of Omega‐3 Fatty Acid Alone and in Combination with Proprietary Chromium Complex on Endothelial Function in Subjects with Metabolic Syndrome: A Randomized, Double‐Blind, Parallel‐Group Clinical Study,\u0026quot; \u003cem\u003eEvidence‐Based Complementary and Alternative Medicine, \u003c/em\u003evol. 2021, no. 1, p. 2972610, 2021.\u003c/li\u003e\n\u003cli\u003eA. P. Avolio, M. Butlin, and A. Walsh, \u0026quot;Arterial blood pressure measurement and pulse wave analysis\u0026mdash;their role in enhancing cardiovascular assessment,\u0026quot; \u003cem\u003ePhysiological measurement, \u003c/em\u003evol. 31, no. 1, p. R1, 2009.\u003c/li\u003e\n\u003cli\u003eL. Xu, M. Q.-H. Meng, C. Shi, K. Wang, and N. Li, \u0026quot;Quantitative analyses of pulse images in Traditional Chinese Medicine,\u0026quot; \u003cem\u003eMedical Acupuncture, \u003c/em\u003evol. 20, no. 3, pp. 175-189, 2008.\u003c/li\u003e\n\u003cli\u003eD. Chambers, C. Huang, and G. Matthews, \u003cem\u003eBasic physiology for anaesthetists\u003c/em\u003e. Cambridge University Press, 2019.\u003c/li\u003e\n\u003cli\u003eE. Agabiti-Rosei\u003cem\u003e et al.\u003c/em\u003e, \u0026quot;Central blood pressure measurements and antihypertensive therapy: a consensus document,\u0026quot; \u003cem\u003eHypertension, \u003c/em\u003evol. 50, no. 1, pp. 154-160, 2007.\u003c/li\u003e\n\u003cli\u003eM. Horiuchi and K. Okita, \u0026quot;Blood flow restricted exercise and vascular function,\u0026quot; \u003cem\u003eInternational journal of vascular medicine, \u003c/em\u003evol. 2012, no. 1, p. 543218, 2012.\u003c/li\u003e\n\u003cli\u003eR. Q. Landers-Ramos, T. Silva, D. A. Dobrosielski, and N. D. Knuth, \u0026quot;Examination of post-exercise microvascular reactivity in healthy adults,\u0026quot; \u003cem\u003eEuropean Journal of Applied Physiology, \u003c/em\u003epp. 1-12, 2025.\u003c/li\u003e\n\u003cli\u003eS. A. Rebello and R. M. van Dam, \u0026quot;Coffee consumption and cardiovascular health: getting to the heart of the matter,\u0026quot; \u003cem\u003eCurrent cardiology reports, \u003c/em\u003evol. 15, no. 10, p. 403, 2013.\u003c/li\u003e\n\u003cli\u003eA. Mahmud and J. Feely, \u0026quot;Acute effect of caffeine on arterial stiffness and aortic pressure waveform,\u0026quot; \u003cem\u003eHypertension, \u003c/em\u003evol. 38, no. 2, pp. 227-231, 2001.\u003c/li\u003e\n\u003cli\u003eS. O\u0026apos;neill and D. Eisner, \u0026quot;A mechanism for the effects of caffeine on Ca2+ release during diastole and systole in isolated rat ventricular myocytes,\u0026quot; \u003cem\u003eThe Journal of physiology, \u003c/em\u003evol. 430, no. 1, pp. 519-536, 1990.\u003c/li\u003e\n\u003cli\u003eS. Noimark\u003cem\u003e et al.\u003c/em\u003e, \u0026quot;Carbon‐nanotube\u0026ndash;PDMS composite coatings on optical fibers for all‐optical ultrasound imaging,\u0026quot; \u003cem\u003eAdvanced Functional Materials, \u003c/em\u003evol. 26, no. 46, pp. 8390-8396, 2016.\u003c/li\u003e\n\u003cli\u003eB.-Y. Hsieh, J. Kim, J. Zhu, S. Li, X. Zhang, and X. Jiang, \u0026quot;A laser ultrasound transducer using carbon nanofibers\u0026ndash;polydimethylsiloxane composite thin film,\u0026quot; \u003cem\u003eApplied Physics Letters, \u003c/em\u003evol. 106, no. 2, 2015.\u003c/li\u003e\n\u003cli\u003eS. H. Lee, Y. Lee, and J. J. Yoh, \u0026quot;Reduced graphene oxide coated polydimethylsiloxane film as an optoacoustic transmitter for high pressure and high frequency ultrasound generation,\u0026quot; \u003cem\u003eApplied Physics Letters, \u003c/em\u003evol. 106, no. 8, 2015.\u003c/li\u003e\n\u003cli\u003eY. Li, Z. Guo, G. Li, and S.-L. Chen, \u0026quot;Miniature fiber-optic high-intensity focused ultrasound device using a candle soot nanoparticles-polydimethylsiloxane composites-coated photoacoustic lens,\u0026quot; \u003cem\u003eOptics express, \u003c/em\u003evol. 26, no. 17, pp. 21700-21711, 2018.\u003c/li\u003e\n\u003cli\u003eS. Noimark\u003cem\u003e et al.\u003c/em\u003e, \u0026quot;Polydimethylsiloxane composites for optical ultrasound generation and multimodality imaging,\u0026quot; \u003cem\u003eAdvanced Functional Materials, \u003c/em\u003evol. 28, no. 9, p. 1704919, 2018.\u003c/li\u003e\n\u003cli\u003eS. Berber, Y.-K. Kwon, and D. Tom\u0026aacute;nek, \u0026quot;Unusually high thermal conductivity of carbon nanotubes,\u0026quot; \u003cem\u003ePhysical review letters, \u003c/em\u003evol. 84, no. 20, p. 4613, 2000.\u003c/li\u003e\n\u003cli\u003eT. Ramanathan, F. Fisher, R. Ruoff, and L. C. Brinson, \u0026quot;Amino-functionalized carbon nanotubes for binding to polymers and biological systems,\u0026quot; \u003cem\u003eChemistry of Materials, \u003c/em\u003evol. 17, no. 6, pp. 1290-1295, 2005.\u003c/li\u003e\n\u003cli\u003eP.-C. Ma, N. A. Siddiqui, G. Marom, and J.-K. Kim, \u0026quot;Dispersion and functionalization of carbon nanotubes for polymer-based nanocomposites: A review,\u0026quot; \u003cem\u003eComposites Part A: Applied Science and Manufacturing, \u003c/em\u003evol. 41, no. 10, pp. 1345-1367, 2010.\u003c/li\u003e\n\u003cli\u003eH. W. Baac\u003cem\u003e et al.\u003c/em\u003e, \u0026quot;Carbon-nanotube optoacoustic lens for focused ultrasound generation and high-precision targeted therapy,\u0026quot; \u003cem\u003eScientific reports, \u003c/em\u003evol. 2, no. 1, p. 989, 2012.\u003c/li\u003e\n\u003cli\u003eJ. Kim, H. Kim, W.-Y. Chang, W. Huang, X. Jiang, and P. A. Dayton, \u0026quot;Candle-soot carbon nanoparticles in photoacoustics: advantages and challenges for laser ultrasound transmitters,\u0026quot; \u003cem\u003eIEEE nanotechnology magazine, \u003c/em\u003evol. 13, no. 3, pp. 13-28, 2019.\u003c/li\u003e\n\u003cli\u003eG. K. Sharma and N. R. James, \u0026quot;Carbon black incorporated carbon nanofiber based polydimethylsiloxane composite for electromagnetic interference shielding,\u0026quot; \u003cem\u003eCarbon Trends, \u003c/em\u003evol. 8, p. 100177, 2022.\u003c/li\u003e\n\u003cli\u003eM. S. Kang, H. Lee, S. J. Jeong, T. J. Eom, J. Kim, and D.-W. Han, \u0026quot;State of the art in carbon nanomaterials for photoacoustic imaging,\u0026quot; \u003cem\u003eBiomedicines, \u003c/em\u003evol. 10, no. 6, p. 1374, 2022.\u003cstrong\u003e\u003c/strong\u003e\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"College of Physics and Optoelectronic Engineering, Jinan University","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Wearable devices, Optoacoustics, Fiber-optic sensors, MRI compatibility Hemodynamic monitoring","lastPublishedDoi":"10.21203/rs.3.rs-8635823/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8635823/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eContinuous monitoring of arterial waveforms is critical for assessing cardiovascular status in intensive care and intraoperative settings. However, conventional modalities relying on electronics or metallic transducers are strictly incompatible with the strong electromagnetic perturbations in clinical settings, especially magnetic resonance imaging (MRI) suites, creating a blind spot in patient monitoring. Here, we present a fully metal-free, all-fiber optoacoustic system (FOAS) that bridges this gap by integrating focused optical ultrasound generation with ultrasensitive fiber-laser detection in a compact wearable platform. This architecture enables beat-to-beat reconstruction of blood pressure waveforms with high fidelity, preserving morphological features essential for vascular compliance analysis. In-vivo validations, including physiological perturbations (exercise, caffeine) and measurements across 10 healthy volunteers, demonstrated the system's robustness in tracking hemodynamic dynamics and resolving inter-subject waveform variability (e.g., systolic-to-dicrotic notch interval). Crucially, the artifact-free operation was demonstrated inside an active 3T MRI scanner, confirming superior electromagnetic immunity. This work establishes fiber optoacoustic as a transformative platform for ambulatory hemodynamic monitoring, extending precise cardiovascular profiling into electromagnetically constrained clinical environments.\u003c/p\u003e","manuscriptTitle":"A Wearable Fiber-Optoacoustic Interface for Continuous Deep-Tissue Hemodynamics in MRI Environments","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-01-23 06:26:22","doi":"10.21203/rs.3.rs-8635823/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"19172cb8-1c80-4ad0-93e0-1fd5992cbaeb","owner":[],"postedDate":"January 23rd, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":61341486,"name":"Optics/Lasers"}],"tags":[],"updatedAt":"2026-01-23T06:26:22+00:00","versionOfRecord":[],"versionCreatedAt":"2026-01-23 06:26:22","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8635823","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8635823","identity":"rs-8635823","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}